On Wed, Dec 16 2009 at 3:39pm -0500, Mike Snitzer <snitzer@xxxxxxxxxx> wrote: > On Wed, Dec 16 2009 at 3:05am -0500, > Mikulas Patocka <mpatocka@xxxxxxxxxx> wrote: ... > > - Exposed interface for snapshots-of-snapshots, tested that they work > > Where is that interface documented? I see the 'dmsetup message ...' to do so is quietly mentioned in the "Create new snapshot" section of the documentation. > As an aside, I have some ideas for improving > Documentation/device-mapper/dm-multisnapshot.txt > I'll just send a patch and we can go from there. The inlined [TODO: ...] comments in the following patch are things I'm looking to understand by reviewing the code; but I think they should be answered in the Documentation. I wanted to get you my edits sooner rather than later. diff --git a/Documentation/device-mapper/dm-multisnapshot.txt b/Documentation/device-mapper/dm-multisnapshot.txt index 0dff16e..d730d3a 100644 --- a/Documentation/device-mapper/dm-multisnapshot.txt +++ b/Documentation/device-mapper/dm-multisnapshot.txt @@ -1,64 +1,90 @@ -This snapshot implementation has shared storage and high number of snapshots. +Device-mapper multiple snapshot support +======================================= -The work is split to two modules: -dm-multisnapshot.ko - the general module -dm-store-mikulas.ko - the snapshot store +Device-mapper allows a single copy-on-write (COW) block device to be +shared among multiple snapshots of an origin device. This variant of dm +snapshot is ideal for supporting high numbers of snapshots. It also +supports snapshots of snapshots. -The modularity allows to load other snapshot stores. +There is a single dm target: +multisnapshot -Usage: -Create two logical volumes, one for origin and one for snapshots. -(assume /dev/mapper/vg1-lv1 for origin and /dev/mapper/vg1-lv2 for snapshot in -these examples) +and associated shared COW storage modules: +mikulas +daniel -Clear the first sector of the snapshot volume: -dd if=/dev/zero of=/dev/mapper/vg1-lv2 bs=4096 count=1 +[TODO: expand on the benefits/design of each store; so as to help a user + decide between them?] + +*) multisnapshot <origin> <COW device> <chunksize> + <# generic args> <generic args> <shared COW store type> + <# shared COW store args> <shared COW store args> + [<# snapshot ids> <snapshot ids>] Table line arguments: -- origin device -- shared store device -- chunk size -- number of generic arguments -- generic arguments +- <origin> : origin device +- <COW device> : shared COW store device +- <chunksize> : chunk size +- <# generic args> : number of generic arguments +- <generic args> : generic arguments sync-snapshots --- synchronize snapshots according to the list preserve-on-error --- halt the origin on error in the snapshot store -- shared store type -- number of arguments for shared store type -- shared store arguments +- <shared COW store type> : shared COW store type + mikulas --- TODO + daniel --- TODO +- <# shared COW store args> : number of arguments for shared COW store type +- <shared COW store args> : shared COW store arguments cache-threshold size --- a background write is started cache-limit size --- a limit for metadata cache size -if sync-snapshots was specified - - number of snapshot ids - - snapshot ids +If 'sync-snapshots' was specified: +- <# snapshot ids> : number of snapshot ids +- <snapshot ids> : snapshot ids in desired sync order + + +Usage +===== +*) Create two logical volumes, one for origin and one for snapshots. +(The following examples assume /dev/mapper/vg1-lv1 for origin and +/dev/mapper/vg1-lv2 for snapshot) + +*) Clear the first 4 sectors of the snapshot volume: +[TODO: I see below that the store will create a new metadata structure if +the snapshot device were zero'd. What if it wasn't zero'd and the +device still has data? Appears the ctr will error out. So will lvm +blindly zero any device that it is told to use for a multisnap store?] +dd if=/dev/zero of=/dev/mapper/vg1-lv2 bs=4096 count=1 -Load the shared snapshot driver: +*) Load the shared snapshot driver: echo 0 `blockdev --getsize /dev/mapper/vg1-lv1` multisnapshot /dev/mapper/vg1-lv1 /dev/mapper/vg1-lv2 16 0 mikulas 0|dmsetup create ms -(16 is the chunk size in 512-byte sectors. You can place different number there) -This creates the origin store on /dev/mapper/ms. If the store was zeroed, it -creates new structure, otherwise it loads existing structure. +(16 is the chunk size in 512-byte sectors. You can place a different +number there. [TODO: what is the limit?]) +This creates the origin store on /dev/mapper/ms. If the COW store was +zeroed, it creates new structure, otherwise it loads existing structure. Once this is done, you should no longer access /dev/mapper/vg1-lv1 and /dev/mapper/vg1-lv2 and only use /dev/mapper/ms. -Create new snapshot: +*) Create new snapshot: +[TODO: what is the '0' in the following messages?] dmsetup message /dev/mapper/ms 0 create - If you want to create snapshot-of-snapshot, use + If you want to create snapshot-of-snapshot, use: dmsetup message /dev/mapper/ms 0 create_subsnap <snapID> dmsetup status /dev/mapper/ms - (this will find out the newly created snapshot ID) + (this will display the newly created snapshot ID) + [TODO: how will that scale? Does the status output sort based on + creation time? maybe show example output?] dmsetup suspend /dev/mapper/ms dmsetup resume /dev/mapper/ms -Attach the snapshot: -echo 0 `blockdev --getsize /dev/mapper/vg1-lv1` multisnap-snap /dev/mapper/vg1-lv1 0|dmsetup create ms0 -(that '0' is the snapshot id ... you can use different number) -This attaches the snapshot '0' on /dev/mapper/ms0 +*) Attach the snapshot: +echo 0 `blockdev --getsize /dev/mapper/vg1-lv1` multisnap-snap /dev/mapper/vg1-lv1 <snapID>|dmsetup create ms0 +This attaches the snapshot with <snapID> to /dev/mapper/ms0 -Delete the snapshot: -dmsetup message /dev/mapper/ms 0 delete 0 -(the parameter after "delete" is the snapshot id) +*) Delete the snapshot: +dmsetup message /dev/mapper/ms 0 delete <snapID> -See status: +*) See status: +[TODO: could use some further cleanup.. maybe show example output?] dmsetup status prints these information about the multisnapshot device: - number of arguments befor the snapshot id list (5) - 0 on active storage, -error number on error (-ENOSPC, -EIO, etc.) @@ -69,9 +95,9 @@ dmsetup status prints these information about the multisnapshot device: - a number of snapshots - existing snapshot IDs -Unload it: +*) Unload it: dmsetup remove ms dmsetup remove ms0 ... etc. (note, once you unload the origin, the snapshots become inaccessible -- the devices exist but they return -EIO on everything) +- the devices exist but they return -EIO when accessed) -- dm-devel mailing list dm-devel@xxxxxxxxxx https://www.redhat.com/mailman/listinfo/dm-devel
