[LU-13417] DNE3: mkdir() automatically create remote directory on MDS which has more space

• Type: Improvement
• Resolution: Fixed
• Priority: Major
• Fix Version/s: Lustre 2.15.0
• Affects Version/s: Lustre 2.13.0
• Labels: dne3
• Severity: 3
• Assignee: Lai Siyao
• Reporter: Andreas Dilger

Description

Since the patches from LU-11213 landed for 2.13.0, I thought "lfs setdirstripe -i -1 /mnt/lustre" on the root or other existing directory would allow creation of remote directories on other MDTs using plain "mkdir" commands. This is different than the case of "lfs setdirstripe -i -1 -c N /mnt/lustre" selecting stripes on less-full MDTs that was landed via patch https://review.whamcloud.com/35825 "LU-12624 lod: alloc dir stripes by QoS", but this patch also removed the "space" hash, so I thought that regular mkdir of a directory could be allowed to balance across MDTs?

However, I can't seem to get this to work. On current master (2.13.52-259, just before 2.13.53) I'm not able to use "lfs setdirstripe -i -1 /path/to/dir" on an existing directory. It seems to select the less-full MDT if I explicitly run "lfs mkdir -i -1" for a new directory, but that was also true in 2.12 using patch https://review.whamcloud.com/30598 "LU-10277 utils: 'lfs mkdir -i -1' pick the less full MDTs", so it isn't clear how to enable the LU-11213 functionality to balance directories across MDTs?

There should be a way for "mkdir(2)" from a normal application (not "lfs mkdir -i -1") to be able to create remote (1-stripe) directories in the filesystem, and it should be possible to set this by default on the root directory (per LU-11213). This is critical for being able to use multiple MDTs effectively without users knowing the details of how to configure striped/remote directories manually, or be forced to set all directories as striped (unwelcome due to performance overhead).

The default mdt_qos_threshold_rr value should be reduced significantly (e.g. 1-2% and/or modified so that some amount of MDT balancing is active when the filesystem is balanced, at least in the root directory by default. Otherwise, without users understanding the details of DNE MDT0000 will hold all of the inodes, when it would be better if the top 1 or 2 levels of directories should be distributed across MDTs.

Maybe this is mostly a documentation issue, and the "lfs-setdirstripe.1" man page needs to be updated to be more clear so I can understand what needs to be done to enable this? (also the usage message for setdirstripe/mkdir should remove the "This can only be done on MDT0 with the right of administrator" message.)

Issue Links

• is duplicated by: LU-10784 DNE3: mkdir() automatically create remote directory on MDS which has more space
• is related to: LU-17300 Avoid creating new dir/file/object on newly added MDT/OST
• is related to: LU-13439 DNE3: MDT QOS tuning to avoid full MDTs completely
• is related to: LU-14898 sanity test_413a: (max - min) * 100 / min: division by 0 (error token is "min")
• is related to: LU-14909 LU-13417 patch breaks few recovery tests
• is related to: LU-17334 Client should handle dir/file/object created on newly added MDT/OST
• is related to: LU-18343 interop: sanity test_160a: FAIL: mkdir parent '/mnt/lustre/d160a.sanity/pics/2008' failed
• is related to: LU-14792 DNE3: enable filesystem-wide default LMV
• is related to: LU-15856 "lfs setdirstripe -D ... <dir>" should create directory if missing
• is related to: LU-13440 DNE3: limit directory default layout inheritance
• is related to: LU-13560 'lfs mkdir -i N' should be 'sticky' on a particular MDT
• is related to: LU-15216 improve MDT QOS space balance
• is related to: LU-11025 DNE3: directory restripe
• is related to: LU-12624 DNE3: striped directory allocate stripes by QoS
• is related to: LU-11213 DNE3: remote mkdir() in ROOT/ by default
• is related to: LU-15850 MDT QOS should always be used for round-robin directories.

Comments

[Andreas Dilger]

I was looking at the LMV code and tested to verify that the LU-11213 implementation of "lmv_create()" will already do round-robin allocation across MDTs in a directory with "-D -c 1 -i -1" set, and will only use QOS weight-balanced MDT selection if the MDT space imbalance is over the qos_threshold_rr limit. This is enabled only when a default directory layout is set on the root directory. It will also only be inherited one level down, because lum_stripe_count=1 default layouts are changed to lum_stripe_count=0 in lod_ah_init(), which is no longer considered to be inherited.

I think that means there is a (hopefully simple) change that can be done to make this functionality more useful for filesystems:

• explicitly set "trusted.dmv = .lum_magic=LMV_USER_MAGIC, .lum_stripe_count=1, .lum_stripe_index=-1" xattr in mdd_prepare() for newly formatted filesystems
• one disadvantage is that this will only be inherited for the top-level directory, and would need LU-13440 to be inherited for 2-3 levels
• another disadvantage is that this needs to be enabled manually by the user on "ROOT/" for existing filesystems

While I think this will not be perfect, it will be a lot better than defaulting to not using all of the other MDTs unless the user knows to explicitly use "lfs mkdir" and/or "lfs setdirstripe -D" on the filesystem to start using other MDTs.

[Andreas Dilger]

Peter, this still needs some work to make the remote MDT selection heuristics a bit better.

For top-level directories, it makes sense to round-robin them when the MDTs are relatively empty, and only pick a specific MDT when they are imbalanced.

Also, I think the free space threshold needs to be smaller for MDT imbalance than for OST imbalance.

[Peter Jones]

Landed for 2.14

[Gerrit Updater]

Oleg Drokin ([e-mail]) merged in patch https://review.whamcloud.com/38160/
Subject: LU-13417 utils: lfs setdirstripe -D -i -1 should work
Project: fs/lustre-release
Branch: master
Current Patch Set:
Commit: eeab6942a8dc65dab789c7ca85cc31ba5cee74f3

[Gerrit Updater]

Andreas Dilger ([e-mail]) uploaded a new patch: https://review.whamcloud.com/38160
Subject: LU-13417 utils: lfs setstripe -D -i -1 should work
Project: fs/lustre-release
Branch: master
Current Patch Set: 1
Commit: 8da6905fb197ef226ca091f53335189f243bbcbe

[Andreas Dilger]

It's because 'lfs setdirstripe -D -i -1 <dir>' is used to delete default stripe... You need to use 'lfs setdirstripe -D -i -1 -c 1' to enable balanced subdirectories creation for plain directories.

I think this is a user interface bug then, or a bug in how LOD is interpreting the layout. In my testing, "lfs getdirstripe" showed "lmv_stripe_count:0 lmv_stripe_offset:-1 lmv_hash_type:none" was set on the directory, but it didn't affect the creation of directories with "mkdir()".

I would expect "lfs setdirstripe -d -D $dir" to delete the default layout for a directory, which seems to work, with "-d" already implying "-D" internally, but it is non-obvious because "lmv_stripe_offset:-1" is actually the default value, so "deleting" this layout didn't help.

I would also expect "lfs setdirstripe -D -i -1" to set the default layout to create remote directories, matching how "lfs setstripe" works. There were other users confused by this as well. The missing part is that specifying only "-i -1" is internally using the same as "-c 0" which actually results in the existing layout to be reset to the default (local directory creation). I'll push a small patch that makes "-D -i -1" set "-c 1" internally if the stripe count is not specified, so that it doesn't result in unexpected behavior for the user.

Another issue is that the default "qos_threshold_rr=17%" is too high to start balancing directory creation across MDTs. This might mean that MDT0000 is used for many millions of files and top-level directories before any balancing is even started. At that point it will be very difficult to return the balance of the MDTs because so many top-level directories and subdirectories have been created on MDT0000. I think it would be better to start space balancing and/or round-robin MDT selection for root directory entries right away if "lmv_stripe_count:1 lmv_stripe_offset:-1" is set on "ROOT/" (which I think we should make the default for 2.14). If there is only a single MDT then this is no change to behavior, but for multiple MDTs it will start using all MDTs right away at the root level and prevent the MDTs from becoming unbalanced in the first place. If we special-case MDT0000 to be RR/balanced immediately, then a smaller qos_threshold_rr=5% may still be useful to avoid the MDTs becoming too imbalanced, but will be less likely to be needed.

[Lai Siyao]

It's because 'lfs setdirstripe -D -i -1 <dir>' is used to delete default stripe, because when both "mdt_index" and "mdt_count" are unset, it's treated as removal. You need to use 'lfs setdirstripe -D -i -1 -c 1' to enable balanced subdirectories creation for plain directories.