Date: Mon, 09 Dec 2013 13:42:45 -0500 From: Jason Keltz <jas@cse.yorku.ca> To: Matthew Ahrens <mahrens@delphix.com> Cc: freebsd-fs <freebsd-fs@freebsd.org> Subject: Re: question about zfs written property Message-ID: <52A60F25.8040800@cse.yorku.ca> In-Reply-To: <CAJjvXiEO8u0Y2Y%2Be_XBKwFZ6oF8u5qGL7k9NfKbvWypnTG0mhw@mail.gmail.com> References: <52A5F80D.8020206@cse.yorku.ca> <CAJjvXiEO8u0Y2Y%2Be_XBKwFZ6oF8u5qGL7k9NfKbvWypnTG0mhw@mail.gmail.com>
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On 12/09/2013 01:24 PM, Matthew Ahrens wrote: > Jason, I'm cc'ing the freebsd mailing list as well. In general that > is a better forum for questions about how to use ZFS. > Thanks.. > > On Mon, Dec 9, 2013 at 9:04 AM, Jason Keltz <jas@cse.yorku.ca > <mailto:jas@cse.yorku.ca>> wrote: > > Hi.. > > I saw your names on the feature addition in illumos for the > "written" property for ZFS: > > https://www.illumos.org/issues/1645 > > I had a question and was hoping you might have a moment to answer. > > I'm rsyncing data from a Linux-based system to a ZFS backup and > archive server. It's running FreeBSD, but it's the same ZFS code > base as illumos. I'm seeing (what I think are) some weird numbers > looking at ZFS written property ... > > For example: > > Sat Dec 7 01:05:00 EST 2013 sync start > rsync://backup@forest-mrpriv/home9 /local/backup/home9 > (189G/264G/1.41x) > Sat Dec 7 01:33:20 EST 2013 sync finish > rsync://backup@forest-mrpriv/home9 /local/backup/home9 > (190G/265G/1.41x) > Sat Dec 7 01:33:20 EST 2013 sync elapsed 00h:28m:20s > rsync://backup@forest-mrpriv/home9 /local/backup/home9, 514M > unarchived > Sat Dec 7 06:32:58 EST 2013 archive create home9 daily 20131207 > as pool1/backup/home9@20131207, 518M > > In the third line, where you see "514M unarchived", I write out > the property of "written" after the rsync completes. However, > when the archive (just a snapshot) runs (hours later), there's 4 > MB more data!? Nothing touches the data after the rsync completes. > Both lines are probing the same property on the same dataset. > How can they get a different result? > > > If you are getting the "written" property just after the rsync > completes, it's possible that there is still some data "in flight" > inside ZFS. If you run "sync", that should flush out all the dirty > data and update the space accounting. Unfortunately this is only > documented in the description of the "used" property, we should add > similar qualifiers to "available", "referenced", "written", > "logicalreferenced", etc.: > > The amount of space used, available, or referenced does > > not take into account pending changes. Pending changes > > are generally accounted for within a few seconds. Com- > > mitting a change to a disk using fsync(3c) or O_SYNC > > does not necessarily guarantee that the space usage > > information is updated immediately. > > Yes. I repeated the test. It takes an additional 3 seconds after my 6 MB rsync completes for "written" to produce the correct result. The result is repeatable. It doesn't appear that running "sync" actually reduces the delay. A sleep() works though :) (I know I could sure use one!) Jason.
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