Bob Friesenhahn wrote: > On Fri, 25 Jul 2008, Brandon High wrote: > >> Mirroring will provide a boost on reads, since the system to read from >> both sides of the mirror. It will not provide an increase on writes, >> since the system needs to wait for both halves of the mirror to >> finish. It could be slightly slower than a single raid5. >> > > Is anything really slower than a RAID5? >
touche' :-) > I am not sure if ZFS really has to wait for both sides of a mirror to > finish, but even if it does, if there are enough VDEVs then ZFS can > still proceed with writing. > > This is actually an interesting question. Lets put it in context. In the bad old days, logical volume managers (eg SVM) sat between a file system and physical disks. When the file system wrote a block to the pseudo-device, SVM would wait until the data protection mechanism had committed before returning to the file system. For mirrors, this meant that both sides of the mirror had committed to disk. For RAID-5, it meant that the data and its parity had been updated, which may have required read-modify-write cycles. ZFS is slightly different. ZFS will queue I/Os to devices with the redundant data being just another entry in the queue. When the transaction group commits, ZFS flushes everything to persistent storage. In the mirror case, each side of the mirror will be processing a queue and occasionally will be asked to flush. For normal writes, these won't seem very different. For synchronous (blocking) writes, they will be different. In the SVM case, the blocking write will wait until both sides of the mirror have been written. For ZFS, the blocking write will wait until the write has been committed in the ZIL. If you do not use a separate ZIL log, then ZFS will seem similar to the SVM case. But if you do use a separate ZIL log, then you can take advantage of the various write-latency optimized storage devices. You should be able to see a big win for ZFS over the SVM case, especially for JBODs. -- richard _______________________________________________ zfs-discuss mailing list zfs-discuss@opensolaris.org http://mail.opensolaris.org/mailman/listinfo/zfs-discuss
