Thanks for the explanation folks.
So if I cannot get Apache/Webdav to write synchronously, (and it does
not look like I can), then is it possible to tune the ARC to be more
write-buffered heavy?
My biggest problem is with very quick spikes in writes periodically
throughout the day. If I were able to buffer these better, I would be in
pretty good shape. The machines are already (economically) maxed out on
ram at 32 gigs.
If I were to add in the SSD L2ARC devices for read caching, can I
configure the ARC to give up some of it's read caching for more write
buffering?
Thanks.
Neil Perrin wrote:
Patrick,
The ZIL is only used for synchronous requests like O_DSYNC/O_SYNC and
fsync(). Your iozone command must be doing some synchronous writes.
All the other tests (dd, cat, cp, ...) do everything asynchronously.
That is they do not require the data to be on stable storage on
return from the write. So asynchronous writes get cached in memory
(the ARC) and written out periodically (every 30 seconds or less)
when the transaction group commits.
The ZIL would be heavily used if your system were a NFS server.
Databases also do synchronous writes.
Neil.
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