[zfs-discuss] Re: Efficiency when reading the same file blocks
Given your question are you about to come back with a case where you are not seeing this? As a follow-up, I tested this on UFS and ZFS. UFS does very poorly: the I/O rate drops off quickly when you add processes while reading the same blocks from the same file at the same time. I don't know why this is, and it would be helpful if someone explained it to me. ZFS did a lot better. There did not appear to be any drop-off after the first process. There was a drop in I/O rate as I kept adding processes, but in that case the CPU was at 100%. I haven't had a chance to test this on a bigger box, but I suspect ZFS is able to keep the sequential read going at full speed (at least if the blocks happen to be written sequentially). I did these tests with each process being a dd if=bigfile of=/dev/null started at the same time, and I measured I/O rate with zpool iostat mypool 2 and iostat -Md 2. This message posted from opensolaris.org ___ zfs-discuss mailing list zfs-discuss@opensolaris.org http://mail.opensolaris.org/mailman/listinfo/zfs-discuss
[zfs-discuss] Re: Re: Efficiency when reading the same file blocks
On February 26, 2007 9:05:21 AM -0800 Jeff Davis But you have to be aware that logically sequential reads do not necessarily translate into physically sequential reads with zfs. zfs I understand that the COW design can fragment files. I'm still trying to understand how that would affect a database. It seems like that may be bad for performance on single disks due to the seeking, but I would expect that to be less significant when you have many spindles. I've read the following blogs regarding the topic, but didn't find a lot of details: http://blogs.sun.com/bonwick/entry/zfs_block_allocation http://blogs.sun.com/realneel/entry/zfs_and_databases This message posted from opensolaris.org ___ zfs-discuss mailing list zfs-discuss@opensolaris.org http://mail.opensolaris.org/mailman/listinfo/zfs-discuss
[zfs-discuss] Re: Efficiency when reading the same file blocks
Given your question are you about to come back with a case where you are not seeing this? Actually, the case where I saw the bad behavior was in Linux using the CFQ I/O scheduler. When reading the same file sequentially, adding processes drastically reduced total disk throughput (single disk machine). Using the Linux anticipatory scheduler worked just fine: no additional I/O costs for more processes. That got me worried about the project I'm working on, and I wanted to understand ZFS's caching behavior better to prove to myself that the problem wouldn't happen under ZFS. Clearly the block will be in cache on the second read, but what I'd like to know is if ZFS will ask the disk to do a long, efficient sequential read of the disk, or whether it will somehow not recognize that the read is sequential because the requests are coming from different processes? This message posted from opensolaris.org ___ zfs-discuss mailing list zfs-discuss@opensolaris.org http://mail.opensolaris.org/mailman/listinfo/zfs-discuss
[zfs-discuss] Efficiency when reading the same file blocks
if you have N processes reading the same file sequentially (where file size is much greater than physical memory) from the same starting position, should I expect that all N processes finish in the same time as if it were a single process? In other words, if you have one process that reads blocks from a file, is it free (meaning no additional total I/O cost) to have another process read the same blocks from the same file at the same time? This message posted from opensolaris.org ___ zfs-discuss mailing list zfs-discuss@opensolaris.org http://mail.opensolaris.org/mailman/listinfo/zfs-discuss
