man: document allocation process in lvm.8

[lvm2.git] / doc / lvm2-raid.txt

=======================
= LVM RAID Design Doc =
=======================

#############################
# Chapter 1: User-Interface #
#############################

***************** CREATING A RAID DEVICE ******************

01: lvcreate --type <RAID type> \
02:          [--regionsize <size>] \
03:          [-i/--stripes <#>] [-I,--stripesize <size>] \
04:          [-m/--mirrors <#>] \
05:          [--[min|max]recoveryrate <kB/sec/disk>] \
06:          [--stripecache <size>] \
07:          [--writemostly <devices>] \
08:          [--maxwritebehind <size>] \
09:          [[no]sync] \
10:          <Other normal args, like: -L 5G -n lv vg> \
11:          [devices]

Line 01:
I don't intend for there to be shorthand options for specifying the
segment type.  The available RAID types are:
        "raid0"  - Stripe [NOT IMPLEMENTED]
        "raid1"  - should replace DM Mirroring
        "raid10" - striped mirrors, [NOT IMPLEMENTED]
        "raid4"  - RAID4
        "raid5"  - Same as "raid5_ls" (Same default as MD)
        "raid5_la" - RAID5 Rotating parity 0 with data continuation
        "raid5_ra" - RAID5 Rotating parity N with data continuation
        "raid5_ls" - RAID5 Rotating parity 0 with data restart
        "raid5_rs" - RAID5 Rotating parity N with data restart
        "raid6"    - Same as "raid6_zr"
        "raid6_zr" - RAID6 Rotating parity 0 with data restart
        "raid6_nr" - RAID6 Rotating parity N with data restart
        "raid6_nc" - RAID6 Rotating parity N with data continuation
The exception to 'no shorthand options' will be where the RAID implementations
can displace traditional tagets.  This is the case with 'mirror' and 'raid1'.
In this case, "mirror_segtype_default" - found under the "global" section in
lvm.conf - can be set to "mirror" or "raid1".  The segment type inferred when
the '-m' option is used will be taken from this setting.  The default segment
types can be overridden on the command line by using the '--type' argument.

Line 02:
Region size is relevant for all RAID types.  It defines the granularity for
which the bitmap will track the active areas of disk.  The default is currently
4MiB.  I see no reason to change this unless it is a problem for MD performance.
MD does impose a restriction of 2^21 regions for a given device, however.  This
means two things: 1) we should never need a metadata area larger than
8kiB+sizeof(superblock)+bitmap_offset (IOW, pretty small) and 2) the region
size will have to be upwardly revised if the device is larger than 8TiB
(assuming defaults).

Line 03/04:
The '-m/--mirrors' option is only relevant to RAID1 and will be used just like
it is today for DM mirroring.  For all other RAID types, -i/--stripes and
-I/--stripesize are relevant.  The former will specify the number of data
devices that will be used for striping.  For example, if the user specifies
'--type raid0 -i 3', then 3 devices are needed.  If the user specifies
'--type raid6 -i 3', then 5 devices are needed.  The -I/--stripesize may be
confusing to MD users, as they use the term "chunksize".  I think they will
adapt without issue and I don't wish to create a conflict with the term
"chunksize" that we use for snapshots.

Line 05/06/07:
I'm still not clear on how to specify these options.  Some are easier than
others.  '--writemostly' is particularly hard because it involves specifying
which devices shall be 'write-mostly' and thus, also have 'max-write-behind'
applied to them.  It has been suggested that a '--readmostly'/'--readfavored'
or similar option could be introduced as a way to specify a primary disk vs.
specifying all the non-primary disks via '--writemostly'.  I like this idea,
but haven't come up with a good name yet.  Thus, these will remain
unimplemented until future specification.

Line 09/10/11:
These are familiar.

Further creation related ideas:
Today, you can specify '--type mirror' without an '-m/--mirrors' argument
necessary.  The number of devices defaults to two (and the log defaults to
'disk').  A similar thing should happen with the RAID types.  All of them
should default to having two data devices unless otherwise specified.  This
would mean a total number of 2 devices for RAID 0/1, 3 devices for RAID 4/5,
and 4 devices for RAID 6/10.


***************** CONVERTING A RAID DEVICE ******************

01: lvconvert [--type <RAID type>] \
02:           [-R/--regionsize <size>] \
03:           [-i/--stripes <#>] [-I,--stripesize <size>] \
04:           [-m/--mirrors <#>] \
05:           [--merge]
06:           [--splitmirrors <#> [--trackchanges]] \
07:           [--replace <sub_lv|device>] \
08:           [--[min|max]recoveryrate <kB/sec/disk>] \
09:           [--stripecache <size>] \
10:           [--writemostly <devices>] \
11:           [--maxwritebehind <size>] \
12:           vg/lv
13:           [devices]

lvconvert should work exactly as it does now when dealing with mirrors -
even if(when) we switch to MD RAID1.  Of course, there are no plans to
allow the presense of the metadata area to be configurable (e.g. --corelog).
It will be simple enough to detect if the LV being up/down-converted is
new or old-style mirroring.

If we choose to use MD RAID0 as well, it will be possible to change the
number of stripes and the stripesize.  It is therefore conceivable to see
something like, 'lvconvert -i +1 vg/lv'.

Line 01:
It is possible to change the RAID type of an LV - even if that LV is already
a RAID device of a different type.  For example, you could change from
RAID4 to RAID5 or RAID5 to RAID6.

Line 02/03/04:
These are familiar options - all of which would now be available as options
for change.  (However, it'd be nice if we didn't have regionsize in there.
It's simple on the kernel side, but is just an extra - often unecessary -
parameter to many functions in the LVM codebase.)

Line 05:
This option is used to merge an LV back into a RAID1 array - provided it was
split for temporary read-only use by '--splitmirrors 1 --trackchanges'.

Line 06:
The '--splitmirrors <#>' argument should be familiar from the "mirror" segment
type.  It allows RAID1 images to be split from the array to form a new LV.
Either the original LV or the split LV - or both - could become a linear LV as
a result.  If the '--trackchanges' argument is specified in addition to
'--splitmirrors', an LV will be split from the array.  It will be read-only.
This operation does not change the original array - except that it uses an empty
slot to hold the position of the split LV which it expects to return in the
future (see the '--merge' argument).  It tracks any changes that occur to the
array while the slot is kept in reserve.  If the LV is merged back into the
array, only the changes are resync'ed to the returning image.  Repeating the
'lvconvert' operation without the '--trackchanges' option will complete the
split of the LV permanently.

Line 07:
This option allows the user to specify a sub_lv (e.g. a mirror image) or
a particular device for replacement.  The device (or all the devices in
the sub_lv) will be removed and replaced with different devices from the
VG.

Line 08/09/10/11:
It should be possible to alter these parameters of a RAID device.  As with
lvcreate, however, I'm not entirely certain how to best define some of these.
We don't need all the capabilities at once though, so it isn't a pressing
issue.

Line 12:
The LV to operate on.

Line 13:
Devices that are to be used to satisfy the conversion request.  If the
operation removes devices or splits a mirror, then the devices specified
form the list of candidates for removal.  If the operation adds or replaces
devices, then the devices specified form the list of candidates for allocation.



###############################################
# Chapter 2: LVM RAID internal representation #
###############################################

The internal representation is somewhat like mirroring, but with alterations
for the different metadata components.  LVM mirroring has a single log LV,
but RAID will have one for each data device.  Because of this, I've added a
new 'areas' list to the 'struct lv_segment' - 'meta_areas'.  There is exactly
a one-to-one relationship between 'areas' and 'meta_areas'.  The 'areas' array
still holds the data sub-lv's (similar to mirroring), while the 'meta_areas'
array holds the metadata sub-lv's (akin to the mirroring log device).

The sub_lvs will be named '%s_rimage_%d' instead of '%s_mimage_%d' as it is
for mirroring, and '%s_rmeta_%d' instead of '%s_mlog'.  Thus, you can imagine
an LV named 'foo' with the following layout:
foo
[foo's lv_segment]
|
|-> foo_rimage_0 (areas[0])
|   [foo_rimage_0's lv_segment]
|-> foo_rimage_1 (areas[1])
|   [foo_rimage_1's lv_segment]
|
|-> foo_rmeta_0 (meta_areas[0])
|   [foo_rmeta_0's lv_segment]
|-> foo_rmeta_1 (meta_areas[1])
|   [foo_rmeta_1's lv_segment]

LVM Meta-data format
====================
The RAID format will need to be able to store parameters that are unique to
RAID and unique to specific RAID sub-devices.  It will be modeled after that
of mirroring.

Here is an example of the mirroring layout:
lv {
        id = "agL1vP-1B8Z-5vnB-41cS-lhBJ-Gcvz-dh3L3H"
        status = ["READ", "WRITE", "VISIBLE"]
        flags = []
        segment_count = 1

        segment1 {
                start_extent = 0
                extent_count = 125      # 500 Megabytes

                type = "mirror"
                mirror_count = 2
                mirror_log = "lv_mlog"
                region_size = 1024

                mirrors = [
                        "lv_mimage_0", 0,
                        "lv_mimage_1", 0
                ]
        }
}

The real trick is dealing with the metadata devices.  Mirroring has an entry,
'mirror_log', in the top-level segment.  This won't work for RAID because there
is a one-to-one mapping between the data devices and the metadata devices.  The
mirror devices are layed-out in sub-device/le pairs.  The 'le' parameter is
redundant since it will always be zero.  So for RAID, I have simple put the
metadata and data devices in pairs without the 'le' parameter.

RAID metadata:
lv {
        id = "EnpqAM-5PEg-i9wB-5amn-P116-1T8k-nS3GfD"
        status = ["READ", "WRITE", "VISIBLE"]
        flags = []
        segment_count = 1

        segment1 {
                start_extent = 0
                extent_count = 125      # 500 Megabytes

                type = "raid1"
                device_count = 2
                region_size = 1024

                raids = [
                        "lv_rmeta_0", "lv_rimage_0",
                        "lv_rmeta_1", "lv_rimage_1",
                ]
        }
}

The metadata also must be capable of representing the various tunables.  We
already have a good example for one from mirroring, region_size.
'max_write_behind', 'stripe_cache', and '[min|max]_recovery_rate' could also
be handled in this way.  However, 'write_mostly' cannot be handled in this
way, because it is a characteristic associated with the sub_lvs, not the
array as a whole.  In these cases, the status field of the sub-lv's themselves
will hold these flags - the meaning being only useful in the larger context.


##############################################
# Chapter 3: LVM RAID implementation details #
##############################################

New Segment Type(s)
===================
I've created a new file 'lib/raid/raid.c' that will handle the various different
RAID types.  While there will be a unique segment type for each RAID variant,
they will all share a common backend - segtype_handler functions and
segtype->flags = SEG_RAID.

I'm also adding a new field to 'struct segment_type', parity_devs.  For every
segment_type except RAID4/5/6, this will be 0.  This field facilitates in
allocation and size calculations.  For example, the lvcreate for RAID5 would
look something like:
~> lvcreate --type raid5 -L 30G -i 3 -n my_raid5 my_vg
or
~> lvcreate --type raid5 -n my_raid5 my_vg /dev/sd[bcdef]1

In the former case, the stripe count (3) and device size are computed, and
then 'segtype->parity_devs' extra devices are allocated of the same size.  In
the latter case, the number of PVs is determined and 'segtype->parity_devs' is
subtracted off to determine the number of stripes.

This should also work in the case of RAID10 and doing things in this manor
should not affect the way size is calculated via the area_multiple.

Allocation
==========
When a RAID device is created, metadata LVs must be created along with the
data LVs that will ultimately compose the top-level RAID array.  For the
foreseeable future, the metadata LVs must reside on the same device as (or
at least one of the devices that compose) the data LV.  We use this property
to simplify the allocation process.  Rather than allocating for the data LVs
and then asking for a small chunk of space on the same device (or the other
way around), we simply ask for the aggregate size of the data LV plus the
metadata LV.  Once we have the space allocated, we divide it between the
metadata and data LVs.  This also greatly simplifies the process of finding
parallel space for all the data LVs that will compose the RAID array.  When
a RAID device is resized, we will not need to take the metadata LV into
account, because it will already be present.

Apart from the metadata areas, the other unique characteristic of RAID
devices is the parity device count.  The number of parity devices does nothing
to the calculation of size-per-device.  The 'area_multiple' means nothing
here.  The parity devices will simply be the same size as all the other devices
and will also require a metadata LV (i.e. it is treated no differently than
the other devices).

Therefore, to allocate space for RAID devices, we need to know two things:
1) how many parity devices are required and 2) does an allocated area need to
be split out for the metadata LVs after finding the space to fill the request.
We simply add these two fields to the 'alloc_handle' data structure as,
'parity_count' and 'alloc_and_split_meta'.  These two fields get set in
'_alloc_init'.   The 'segtype->parity_devs' holds the number of parity
drives and can be directly copied to 'ah->parity_count' and
'alloc_and_split_meta' is set when a RAID segtype is detected and
'metadata_area_count' has been specified.  With these two variables set, we
can calculate how many allocated areas we need.  Also, in the routines that
find the actual space, they stop not when they have found ah->area_count but
when they have found (ah->area_count + ah->parity_count).

Conversion
==========
RAID -> RAID, adding images
---------------------------
When adding images to a RAID array, metadata and data components must be added
as a pair.  It is best to perform as many operations as possible before writing
new LVM metadata.  This allows us to error-out without having to unwind any
changes.  It also makes things easier if the machine should crash during a
conversion operation.  Thus, the actions performed when adding a new image are:
        1) Allocate the required number of metadata/data pairs using the method
           describe above in 'Allocation' (i.e. find the metadata/data space
           as one unit and split the space between them after found - this keeps
           them together on the same device).
        2) Form the metadata/data LVs from the allocated space (leave them
           visible) - setting required RAID_[IMAGE | META] flags as appropriate.
        3) Write the LVM metadata
        4) Activate and clear the metadata LVs.  The clearing of the metadata
           requires the LVM metadata be written (step 3) and is a requirement
           before adding the new metadata LVs to the array.  If the metadata
           is not cleared, it carry residual superblock state from a previous
           array the device may have been part of.
        5) Deactivate new sub-LVs and set them "hidden".
        6) expand the 'first_seg(raid_lv)->areas' and '->meta_areas' array
           for inclusion of the new sub-LVs
        7) Add new sub-LVs and update 'first_seg(raid_lv)->area_count'
        8) Commit new LVM metadata
Failure during any of these steps will not affect the original RAID array.  In
the worst scenario, the user may have to remove the new sub-LVs that did not
yet make it into the array.

RAID -> RAID, removing images
-----------------------------
To remove images from a RAID, the metadata/data LV pairs must be removed
together.  This is pretty straight-forward, but one place where RAID really
differs from the "mirror" segment type is how the resulting "holes" are filled.
When a device is removed from a "mirror" segment type, it is identified, moved
to the end of the 'mirrored_seg->areas' array, and then removed.  This action
causes the other images to shift down and fill the position of the device which
was removed.  While "raid1" could be handled in this way, the other RAID types
could not be - it would corrupt the ordering of the data on the array.  Thus,
when a device is removed from a RAID array, the corresponding metadata/data
sub-LVs are removed from the 'raid_seg->meta_areas' and 'raid_seg->areas' arrays.
The slot in these 'lv_segment_area' arrays are set to 'AREA_UNASSIGNED'.  RAID
is perfectly happy to construct a DM table mapping with '- -' if it comes across
area assigned in such a way.  The pair of dashes is a valid way to tell the RAID
kernel target that the slot should be considered empty.  So, we can remove
devices from a RAID array without affecting the correct operation of the RAID.
(It also becomes easy to replace the empty slots properly if a spare device is
available.)  In the case of RAID1 device removal, the empty slot can be safely
eliminated.  This is done by shifting the higher indexed devices down to fill
the slot.  Even the names of the images will be renamed to properly reflect
their index in the array.  Unlike the "mirror" segment type, you will never have
an image named "*_rimage_1" occupying the index position 0.

As with adding images, removing images holds off on commiting LVM metadata
until all possible changes have been made.  This reduces the likelyhood of bad
intermediate stages being left due to a failure of operation or machine crash.

RAID1 '--splitmirrors', '--trackchanges', and '--merge' operations
------------------------------------------------------------------
This suite of operations is only available to the "raid1" segment type.

Splitting an image from a RAID1 array is almost identical to the removal of
an image described above.  However, the metadata LV associated with the split
image is removed and the data LV is kept and promoted to a top-level device.
(i.e.  It is made visible and stripped of its RAID_IMAGE status flags.)

When the '--trackchanges' option is given along with the '--splitmirrors'
argument, the metadata LV is left as part of the original array.  The data LV
is set as 'VISIBLE' and read-only (~LVM_WRITE).  When the array DM table is
being created, it notices the read-only, VISIBLE nature of the sub-LV and puts
in the '- -' sentinel.  Only a single image can be split from the mirror and
the name of the sub-LV cannot be changed.  Unlike '--splitmirrors' on its own,
the '--name' argument must not be specified.  Therefore, the name of the newly
split LV will remain the same '<lv>_rimage_<N>', where 'N' is the index of the
slot in the array for which it is associated.

When an LV which was split from a RAID1 array with the '--trackchanges' option
is merged back into the array, its read/write status is restored and it is
set as "hidden" again.  Recycling the array (suspend/resume) restores the sub-LV
to its position in the array and begins the process of sync'ing the changes that
were made since the time it was split from the array.

RAID device replacement with '--replace'
----------------------------------------
This option is available to all RAID segment types.

The '--replace' option can be used to remove a particular device from a RAID
logical volume and replace it with a different one in one action (CLI command).
The device device to be removed is specified as the argument to the '--replace'
option.  This option can be specified more than once in a single command,
allowing multiple devices to be replaced at the same time - provided the RAID
logical volume has the necessary redundancy to allow the action.  The devices
to be used as replacements can also be specified in the command; similar to the
way allocatable devices are specified during an up-convert.

Example> lvconvert --replace /dev/sdd1 --replace /dev/sde1 vg/lv /dev/sd[bc]1

RAID '--repair'
---------------
This 'lvconvert' option is available to all RAID segment types and is described
under "RAID Fault Handling".


RAID Fault Handling
===================
RAID is not like traditional LVM mirroring (i.e. the "mirror" segment type).
LVM mirroring required failed devices to be removed or the logical volume would
simply hang.  RAID arrays can keep on running with failed devices.  In fact, for
RAID types other than RAID1 removing a device would mean substituting an error
target or converting to a lower level RAID (e.g. RAID6 -> RAID5, or RAID4/5 to
RAID0).  Therefore, rather than removing a failed device unconditionally, the
user has a couple of options to choose from.

The automated response to a device failure is handled according to the user's
preference defined in lvm.conf:activation.raid_fault_policy.  The options are:
    # "warn"    - Use the system log to warn the user that a device in the RAID
    #             logical volume has failed.  It is left to the user to run
    #             'lvconvert --repair' manually to remove or replace the failed
    #             device.  As long as the number of failed devices does not
    #             exceed the redundancy of the logical volume (1 device for
    #             raid4/5, 2 for raid6, etc) the logical volume will remain
    #             usable.
    #
    # "remove"  - NOT CURRENTLY IMPLEMENTED OR DOCUMENTED IN example.conf.in.
    #             Remove the failed device and reduce the RAID logical volume
    #             accordingly.  If a single device dies in a 3-way mirror,
    #             remove it and reduce the mirror to 2-way.  If a single device
    #             dies in a RAID 4/5 logical volume, reshape it to a striped
    #             volume, etc - RAID 6 -> RAID 4/5 -> RAID 0.  If devices
    #             cannot be removed for lack of redundancy, fail.
    #             THIS OPTION CANNOT YET BE IMPLEMENTED BECAUSE RESHAPE IS NOT
    #             YET SUPPORTED IN linux/drivers/md/dm-raid.c.  The superblock
    #             does not yet hold enough information to support reshaping.
    #
    # "allocate" - Attempt to use any extra physical volumes in the volume
    #             group as spares and replace faulty devices.

If manual intervention is taken, either in response to the automated solution's
"warn" mode or simply because dmeventd hadn't run, then the user can call
'lvconvert --repair vg/lv' and follow the prompts.  They will be prompted
whether or not to replace the device and cause a full recovery of the failed
device.

If replacement is chosen via the manual method or "allocate" is the policy taken
by the automated response, then 'lvconvert --replace' is the mechanism used to
attempt the replacement of the failed device.

'vgreduce --removemissing' is ineffectual at repairing RAID logical volumes.  It
will remove the failed device, but the RAID logical volume will simply continue
to operate with an <unknown> sub-LV.  The user should clear the failed device
with 'lvconvert --repair'.