[#LU-6392] short read/write with stripe count

We are currently hitting an issue on several Lustre filesystems. When we do POSIX IO to/from the Lustre FS, we have a lot of short write/read.

This is easily reproductible with IOR, with one client, one thread, using a transferSize > lustreStripeSize and a lustreStripeCount > 1. However, we were only able to reproduce on 4 of 7 Lustre filesystems.

$ mkdir run_s2
$ lfs setstripe -c 2 -s 1m run_s2/
$ mpirun -n 16  bin/ior  -F -i 4  -t 2m -b 2g  -k -K  -o run_s2/file_s2

IOR-3.0.1: MPI Coordinated Test of Parallel I/O
Began: Thu Mar 19 16:32:39 2015
Command line used: bin/ior -F -i 4 -t 2m -b 2g -k -K -o run_s2/file_s2
Machine: Linux airain1487
Test 0 started: Thu Mar 19 16:32:39 2015
Summary:
    api                = POSIX
    test filename      = run_s2/file_s2
    access             = file-per-process
    ordering in a file = sequential offsets
    ordering inter file= no tasks offsets
    clients            = 16 (16 per node)
    repetitions        = 10
    xfersize           = 2 MiB
    blocksize          = 2 GiB
    aggregate filesize = 32 GiB

access    bw(MiB/s)  block(KiB) xfer(KiB)  open(s)    wr/rd(s)   close(s)   total(s)   iteraccess    bw(MiB/s)  block(KiB) xfer(KiB)  open(s)    wr/rd(s)   close(s)   total(s)   iter
------    ---------  ---------- ---------  --------   --------   --------   --------   ----
write     1961.78    2097152    2048.00    0.017310   16.70      10.10      16.70      0   
read      21425      2097152    2048.00    0.009623   1.53       0.109189   1.53       0   
write     2718.68    2097152    2048.00    0.016170   12.05      4.25       12.05      1   
read      17611      2097152    2048.00    0.010240   1.85       1.45       1.86       1   
write     2973.64    2097152    2048.00    0.023313   11.02      4.05       11.02      2   
read      20168      2097152    2048.00    0.008476   1.62       0.072408   1.62       2   
WARNING: Task 13, partial write(), 1048576 of 2097152 bytes at offset 2038431744 ERRNO=Success
write     2112.16    2097152    2048.00    0.025717   15.51      6.66       15.51      3   
read      12864      2097152    2048.00    0.008304   2.54       2.07       2.55       3

Cluster 1: 480 OST / default stripecount=2 stripesize=1M
Cluster 2: 144 OST / default stripecount=2 stripesize=1M

I did some tests with debug vfstrace, dlmtrace and inode. Logs will be added to this ticket later. (customer is a blacksite)

The short write seems to occur when we lose the layout lock for the file while writing the first stripe. Afterwards, the IO can't continue with the second stripe and the write end.

->> seek to offset 987758592
->> io range 987758592, 989855744 = 2M (transfer size)
->> stripe 942, chunk 987758592, 988807168 = 1M (stripe size)
->> 942 * 1M = 987758592, everything is fine
->> vvp_io_write_start 987758592, 988807168
->> n * vvp_io_commit_write() commits the 4k pages

During the commit, we can observe ldlm_cli_cancel_local(), followed by the message "vvp_conf_set() [0x298c28d7a:0xac:0x0]: losing layout lock". Then come the next stripe.

->> stripe 943, chunk 988807168, 989855744 = 1M (stripe size)
->> no vvp_io_write_start() after because of lock cancellation

[LU-6392] short read/write with stripe count > 1 Created: 20/Mar/15 Updated: 21/Oct/19 Resolved: 26/Mar/15
Status:	Resolved
Project:	Lustre
Component/s:	None
Affects Version/s:	Lustre 2.5.3
Fix Version/s:	None

[LU-6392] short read/write with stripe count > 1 Created: 20/Mar/15 Updated: 21/Oct/19 Resolved: 26/Mar/15