Bob, Thanks for your reply. As you mentioned, adjusting the zfs tunables to reduce the transaction group size yields shorter but more frequent pauses. Unfortunately, this workaround doesn't sufficiently meet our needs (pauses are still too long).
I've reviewed the forum archives and read a number of threads related to this issue. However I didn't find a root-cause explanation for these pauses, only talk of how to ameliorate them. In my particular case, I would like to know why zfs_log_writes are blocked for 180ms on a mutex (seemingly blocked on the intent log itself) when performing zil_itx_assign. Another thread must have a lock on the intent log, no? Overall, the system appears healthy as other system calls (e.g., reads and writes to network devices) complete successfully while writes to the intent log are blocked ... so the problem seems to be access to the zfs intent log. Any additional insight would be appreciated. Thanks On Thu, Feb 25, 2010 at 8:47 PM, Bob Friesenhahn < bfrie...@simple.dallas.tx.us> wrote: > On Thu, 25 Feb 2010, Shane Cox wrote: > > I'm new to ZFS and looking for some assistance with a performance problem: >> >> At the interval of zfs_txg_timeout (I'm using the default of 30), I >> observe 100-200ms >> pauses in my application. Based on my application log files, it appears >> that the >> write() system call is blocked. Digging deeper into the problem with >> DTrace, I >> > > If you check the forum archives you will find quite a few long discussion > threads about this issue. I initiated at least one of them. > > When zfs writes a transaction group there will be a stall in an application > which writes continuously. The main thing you can do is to adjust zfs > tunables to limit the size of a transaction group, or to increase the > frequency of transaction group commits. One such tunable is > > zfs:zfs_write_limit_override > > set in /etc/system. > > Bob > -- > Bob Friesenhahn > bfrie...@simple.dallas.tx.us, http://www.simplesystems.org/users/bfriesen/ > GraphicsMagick Maintainer, > http://www.GraphicsMagick.org/<http://www.graphicsmagick.org/>
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