Jesse,
This isn't a stall -- it's just the natural rhythm of pushing out
transaction groups. ZFS collects work (transactions) until either
the transaction group is full (measured in terms of how much memory
the system has), or five seconds elapse -- whichever comes first.
Your data would seem to suggest that the read side isn't delivering
data as fast as ZFS can write it. However, it's possible that
there's some sort of 'breathing' effect that's hurting performance.
One simple experiment you could try: patch txg_time to 1. That
will cause ZFS to push transaction groups every second instead of
the default of every 5 seconds. If this helps (or if it doesn't),
please let us know.
Thanks,
Jeff
Jesse DeFer wrote:
Hello,
I am having problems with ZFS stalling when writing, any help in
troubleshooting would be appreciated. Every 5 seconds or so the write
bandwidth drops to zero, then picks up a few seconds later (see the zpool
iostat at the bottom of this message). I am running SXDE, snv_55b.
My test consists of copying a 1gb file (with cp) between two drives, one 80GB
PATA, one 500GB SATA. The first drive is the system drive (UFS), the second is
for data. I have configured the data drive with UFS and it does not exhibit
the stalling problem and it runs in almost half the time. I have tried many
different ZFS settings as well: atime=off, compression=off, checksums=off,
zil_disable=1 all to no effect. CPU jumps to about 25% system time during the
stalls, and hovers around 5% when data is being transferred.
# zpool iostat 1
capacity operations bandwidth
pool used avail read write read write
---------- ----- ----- ----- ----- ----- -----
tank 183M 464G 0 17 1.12K 1.93M
tank 183M 464G 0 457 0 57.2M
tank 183M 464G 0 445 0 55.7M
tank 183M 464G 0 405 0 50.7M
tank 366M 464G 0 226 0 4.97M
tank 366M 464G 0 0 0 0
tank 366M 464G 0 0 0 0
tank 366M 464G 0 0 0 0
tank 366M 464G 0 200 0 25.0M
tank 366M 464G 0 431 0 54.0M
tank 366M 464G 0 445 0 55.7M
tank 366M 464G 0 423 0 53.0M
tank 574M 463G 0 270 0 18.1M
tank 574M 463G 0 0 0 0
tank 574M 463G 0 0 0 0
tank 574M 463G 0 0 0 0
tank 574M 463G 0 164 0 20.5M
tank 574M 463G 0 504 0 63.1M
tank 574M 463G 0 405 0 50.7M
tank 753M 463G 0 404 0 42.6M
tank 753M 463G 0 0 0 0
tank 753M 463G 0 0 0 0
tank 753M 463G 0 0 0 0
tank 753M 463G 0 343 0 42.9M
tank 753M 463G 0 476 0 59.5M
tank 753M 463G 0 465 0 50.4M
tank 907M 463G 0 68 0 390K
tank 907M 463G 0 0 0 0
tank 907M 463G 0 11 0 1.40M
tank 907M 463G 0 451 0 56.4M
tank 907M 463G 0 492 0 61.5M
tank 1.01G 463G 0 139 0 7.94M
tank 1.01G 463G 0 0 0 0
Thanks,
Jesse DeFer
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