======================= = LVM RAID Design Doc = ======================= ############################# # Chapter 1: User-Interface # ############################# ***************** CREATING A RAID DEVICE ****************** 01: lvcreate --type \ 02: [--regionsize ] \ 03: [-i/--stripes <#>] [-I,--stripesize ] \ 04: [-m/--mirrors <#>] \ 05: [--[min|max]recoveryrate ] \ 06: [--stripecache ] \ 07: [--writemostly ] \ 08: [--maxwritebehind ] \ 09: [[no]sync] \ 10: \ 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 these cases, a switch will exist in lvm.conf allowing the user to specify which implementation they want. When this is in place, the segment type is inferred from the argument, '-m' for example. 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 ] \ 02: [-R/--regionsize ] \ 03: [-i/--stripes <#>] [-I,--stripesize ] \ 04: [-m/--mirrors <#>] \ 05: [--splitmirrors <#>] \ 06: [--replace ] \ 07: [--[min|max]recoveryrate ] \ 08: [--stripecache ] \ 09: [--writemostly ] \ 10: [--maxwritebehind ] \ 11: vg/lv 12: [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 02/03/04/05: 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 06: 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 07/08/09/10: 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 11: The LV to operate on. Line 12: 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. 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 simply 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).