Thanks for clarifying. If a block is spread across all drives in a RAIDZ group, and there are no partial block reads, how can each drive in the group act like a stripe? Many RAID5&6 implementations can do partial block reads, allowing for parallel random reads across drives (as long as there are no writes in the queue).
Perhaps you are saying that they act like stripes for bandwidth purposes, but not for read ops/sec? -Rob -----Original Message----- From: Bob Friesenhahn [mailto:bfrie...@simple.dallas.tx.us] Sent: Saturday, August 06, 2011 11:41 AM To: Rob Cohen Cc: zfs-discuss@opensolaris.org Subject: Re: [zfs-discuss] Large scale performance query On Sat, 6 Aug 2011, Rob Cohen wrote: > > Can RAIDZ even do a partial block read? Perhaps it needs to read the > full block (from all drives) in order to verify the checksum. > If so, then RAIDZ groups would always act like one stripe, unlike > RAID5/6. ZFS does not do partial block reads/writes. It must read the whole block in order to validate the checksum. If there is a checksum failure, then RAID5 type algorithms are used to produce a corrected block. For this reason, it is wise to make sure that the zfs filesystem blocksize is appropriate for the task, and make sure that the system has sufficient RAM that the zfs ARC can cache enough data that it does not need to re-read from disk for recently accessed files. 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
