samba-bugs at samba.org
2015-Jul-17 14:37 UTC
[Bug 3099] Please parallelize filesystem scan
https://bugzilla.samba.org/show_bug.cgi?id=3099 --- Comment #8 from Chip Schweiss <chip at innovates.com> --- I would argue that optionally all directory scanning should be made parallel. Modern file systems perform best when request queues are kept full. The current mode of rsync scanning directories does nothing to take advantage of this. I currently use scripts to split a couple dozen or so rsync jobs in to literally 100's of jobs. This reduces execution time from what would be days to a couple hours every night. There are lots of scripts like this appearing on the net because the current state of rsync is inadequate. This ticket could reasonably combined with 5124. -- You are receiving this mail because: You are the QA Contact for the bug.
I dont understand - scanning metadata is sped up by thrashing the head all over the disk instead of mostly-sequentially scanning through? How does that work out? /kc On Fri, Jul 17, 2015 at 02:37:21PM +0000, samba-bugs at samba.org said: >https://bugzilla.samba.org/show_bug.cgi?id=3099 > >--- Comment #8 from Chip Schweiss <chip at innovates.com> --- >I would argue that optionally all directory scanning should be made parallel. >Modern file systems perform best when request queues are kept full. The >current mode of rsync scanning directories does nothing to take advantage of >this. > >I currently use scripts to split a couple dozen or so rsync jobs in to >literally 100's of jobs. This reduces execution time from what would be days >to a couple hours every night. There are lots of scripts like this appearing >on the net because the current state of rsync is inadequate. > >This ticket could reasonably combined with 5124. > >-- >You are receiving this mail because: >You are the QA Contact for the bug. > >-- >Please use reply-all for most replies to avoid omitting the mailing list. >To unsubscribe or change options: https://lists.samba.org/mailman/listinfo/rsync >Before posting, read: http://www.catb.org/~esr/faqs/smart-questions.html -- Ken Chase - ken at heavycomputing.ca skype:kenchase23 Toronto Canada Heavy Computing - Clued bandwidth, colocation and managed linux VPS @151 Front St. W.
Sounds to me like maintaining the metadata cache is important - and tuning the filesystem to do so would be more beneficial than caching writes, especially with a backup target where a write already written will likely never be read again (and isnt a big deal if it is since so few files are changed compared to the total # of inodes to scan). Your report of the minutes for the re-sync shows the unthrashed cache is highly valuable. So all we need to do is tune the backup target (and even the operational servers themselves) to maintain more metadata. I dont know how much ram is used per inode, but I'd throw in another 4-8gb just for metadata caching per box, or even more, if it meant scanning was sped up. (Really, actually, one only needs it in the backup target - if you can run all the backups in parallel, and there's N servers to backup, they can all run at 1/N speed, as long as scanning metadata on the backup target is fast enough to keep up with it all -- my total data written is only 20-30GB for example, which at reasonable speed (20-30MB/s even, which is slow) is only 15 minutes total writing. Even 200-300GB changed would be 150 minutes at that rate, and the rate could easily be 4x faster. So, tuning caches to prefer metadata seems to be key. How? As we've discussed before, letting the filesystem at it throws away precious metadata cache, and so tracking your own changes (since the backup system will never be used for anything else, right? :) would be beneficial. Of course the danger is using the backup system for anything else and changing any of the target info - inconsistencies would crop up and make the backup worthless very quickly. /kc On Fri, Jul 17, 2015 at 03:18:02PM +0000, Schweiss, Chip said: >Modern file systems have many internal queues, and service many clients simultaneously. They arrange their work to maximize throughput in both read and write operations. This is the norm on any enterprise file system, be it Hitachi, Oracle, Dell, HP, Isilon, etc. You will get significantly higher throughput if you hit it with multiple threads. These systems have elaborate predictive read ahead caches and perform best when multiple threads hit them. > >Using the test case of a single server with a simple file system such as ext3/4, or xfs, no gains will be seen in multithreading rsync. Use an enterprise file system with 100's of TBs and the more threads you use the faster you will go. Metadata and data on these systems ends up across 100's of disks. Single threads end up severely bound by latency. This is why multi-threading should be optional. It doesn't help everyone. > >For example, one of my rsync jobs moving from a ZFS system in St. Louis, Missouri to a Hitachi HNAS in Minneapolis, Minnesota has over 100 million files. Each day 50 to 100 thousand files get added or updated. A single rsync job would take weeks to parse this job and send the changes. I split it into 120 jobs and it typically completes in 2 hours when no humans are using the systems. A re-sync immediately afterwards, again with 120 jobs, scans both ends in minutes. > >-Chip > >-----Original Message----- >From: rsync [mailto:rsync-bounces at lists.samba.org] On Behalf Of Ken Chase >Sent: Friday, July 17, 2015 9:51 AM >To: samba-bugs at samba.org >Cc: rsync-qa at samba.org >Subject: Re: [Bug 3099] Please parallelize filesystem scan > >I dont understand - scanning metadata is sped up by thrashing the head >all over the disk instead of mostly-sequentially scanning through? > >How does that work out? > >/kc > > >On Fri, Jul 17, 2015 at 02:37:21PM +0000, samba-bugs at samba.org said: > >https://bugzilla.samba.org/show_bug.cgi?id=3099 > > > >--- Comment #8 from Chip Schweiss <chip at innovates.com> --- > >I would argue that optionally all directory scanning should be made parallel. > >Modern file systems perform best when request queues are kept full. The > >current mode of rsync scanning directories does nothing to take advantage of > >this. > > > >I currently use scripts to split a couple dozen or so rsync jobs in to > >literally 100's of jobs. This reduces execution time from what would be days > >to a couple hours every night. There are lots of scripts like this appearing > >on the net because the current state of rsync is inadequate. > > > >This ticket could reasonably combined with 5124. > > > >-- > >You are receiving this mail because: > >You are the QA Contact for the bug. > > > >-- > >Please use reply-all for most replies to avoid omitting the mailing list. > >To unsubscribe or change options: https://lists.samba.org/mailman/listinfo/rsync > >Before posting, read: http://www.catb.org/~esr/faqs/smart-questions.html > >-- >Ken Chase - ken at heavycomputing.ca skype:kenchase23 Toronto Canada >Heavy Computing - Clued bandwidth, colocation and managed linux VPS @151 Front St. W. > >-- >Please use reply-all for most replies to avoid omitting the mailing list. >To unsubscribe or change options: https://lists.samba.org/mailman/listinfo/rsync >Before posting, read: http://www.catb.org/~esr/faqs/smart-questions.html > >________________________________ > >The material in this message is private and may contain Protected Healthcare Information (PHI). If you are not the intended recipient, be advised that any unauthorized use, disclosure, copying or the taking of any action in reliance on the contents of this information is strictly prohibited. If you have received this email in error, please immediately notify the sender via telephone or return mail. -- Ken Chase - ken at heavycomputing.ca skype:kenchase23 +1 416 897 6284 Toronto Canada Heavy Computing - Clued bandwidth, colocation and managed linux VPS @151 Front St. W.
Ken, this just happens to be a special case where your configuration has a huge number of spindles. If you have multiple threads reading the same spindle you'll just be thrashing the heads back & forth. If there is one thread reading at the front of the disk and another thread reading at the end of the disk, it will be *slower* that if you have just one thread reading first the front of the disk and then the end of the disk. Two threads will just have the head whipping back and forth. "one of my rsync jobs moving from a ZFS system ... has over 100 million files" Spreads over how many spindles? The problem is, the optimum way to access the disks depends on how the data lies on the disks. And that's something that a mere program cannot know. Only the filesystem can know that information. Whether it's ext4, md, brtfs, zfs, or whatever -- a program like rsync cannot possibly know how best to access the disk(s) and with how many simultaneous threads. -------------- next part -------------- An HTML attachment was scrubbed... URL: <http://lists.samba.org/pipermail/rsync/attachments/20150717/dbddc1e7/attachment.html>