"Hugh Saunders" <[EMAIL PROTECTED]> writes: > On Sat, May 24, 2008 at 3:21 AM, Richard Elling <[EMAIL PROTECTED]> wrote: >> Consider a case where you might use large, slow SATA drives (1 TByte, >> 7,200 rpm) >> for the main storage, and a single small, fast (36 GByte, 15krpm) drive >> for the >> L2ARC. This might provide a reasonable cost/performance trade-off. > > In this case (or in any other case where a cache device is used), does > the cache improve write performance or only reads? > I presume it cannot increase write performance as the cache is > considered volatile, so the write couldn't be committed until the > data had left the cache device? > >>From the ZFS admin guide [1] "Using cache devices provide the greatest > performance improvement for random read-workloads of mostly static > content." I'm not sure if that means no performance increase for > writes, or just not very much? > > [1]http://docs.sun.com/app/docs/doc/817-2271/gaynr?a=view
My understanding is that this is correct. The writes to the L2ARC are done from non-dirty pages in the in-core ARC, in the background, while nothing is waiting. Dirty pages are not written to the L2ARC at all. AIUI all this is intended to help in the case of flash-based SSDs which tend to have performance that's heavily biased in one direction. Normally, they have outstanding read IOPS and terrible write IOPS, so retiring pages from the RAM ARC to the L2ARC in the background makes sense. There's some good info in the comment here: http://src.opensolaris.org/source/xref/onnv/onnv-gate/usr/src/uts/common/fs/zfs/arc.c#3377 Boyd _______________________________________________ zfs-discuss mailing list zfs-discuss@opensolaris.org http://mail.opensolaris.org/mailman/listinfo/zfs-discuss
