On 4/27/07, Erblichs <[EMAIL PROTECTED]> wrote:
Ming Zhang wrote: > > Hi All > > I wonder if any one have idea about the performance loss caused by COW > in ZFS? If you have to read old data out before write it to some other > place, it involve disk seek. >Ming, Lets take a pro example with a minimal performance tradeoff. All FSs that modify a disk block, IMO, do a full disk block read before anything.
Actually, I'd say that this is the main point that needs to be made. If you're modifying data that was once on disk, that data had to be read from at some point in the past. This is invariably true for any filesystem. With traditional filesystems, that data block is rewritten in the same place. If it were the case that disk blocks were always written immediately after being read, with no intervening I/O to cause a disk seek, COW would have no performance benefit over traditional filesystems. (Well, this isn't true, as there are other benefits to be had.) But it's rarely (if ever) the case that this happens. The modified block is generally written some time after the original block was read, with plenty of intervening I/O that leaves the disk head over some randome location on the platter. So for traditional filesystems, the in-place write of a modified block will typically involve a disk seek. And a second point to be made about this is the effect of caching. With any filesystem, writes are cached in memory and flushed out to disk on a regular basis. With traditional filesystems, flushing the cache involves a set of random writes on the disk, which is possibly going to involve a disk seek for every block written. (In the best case, writes could be reordered in ascending order across the disk to miinimize the disk seeks, but there would still possibly be a small disk seek between each write.) With a COW filesystem, flushing the cache involves writing sequentially to disk with no intervening disk seeks. (This assumes that there's enough free space on disk to avoid fragmentation.) In the ideal case, this means writing to disk at full platter speed. This is where the main performance benefit of COW comes from. Chad Mynhier _______________________________________________ zfs-discuss mailing list zfs-discuss@opensolaris.org http://mail.opensolaris.org/mailman/listinfo/zfs-discuss
