On Wed, 16 Sep 2009, en...@businessgrade.com wrote:
Hi. If I am using slightly more reliable SAS drives versus SATA, SSDs for both L2Arc and ZIL and lots of RAM, will a mirrored pool of say 24 disks hold any significant advantages over a RAIDZ pool?
A mirrored pool will support more IOPs. This is even true when using SSDs for L2Arc and ZIL. Using a SSD for the ZIL dramatically reduces synchronous write latency but the data still needs to be committed to backing store. If the bulk of the synchronous writes are also random writes, then the throughput is still dependent on the IOPs capacity of the backing store. Similarly, more RAM and/or a large SSD L2Arc improves the probability that a repeated read will be retrieved from the ARC rather than the backing store but this depends on the size of the working set, and whether the reads are ever repeated. There are cases (e.g. daily backups) where reads are rarely repeated.
In summary, write IOPs are still write IOPs, and a read cache only works effectively for repeated reads (or reads of recently written data).
You still need to look at the nature of your workload in order to decide if RAIDZ is appropriate.
Bob -- Bob Friesenhahn bfrie...@simple.dallas.tx.us, http://www.simplesystems.org/users/bfriesen/ GraphicsMagick Maintainer, http://www.GraphicsMagick.org/ _______________________________________________ zfs-discuss mailing list zfs-discuss@opensolaris.org http://mail.opensolaris.org/mailman/listinfo/zfs-discuss
