Date: Tue, 8 Dec 2009 17:11:10 -0600 From: Kevin <kevin@your.org> To: Pawel Jakub Dawidek <pjd@FreeBSD.org> Cc: freebsd-fs@freebsd.org Subject: Re: "zfs receive" lock time Message-ID: <EBC441B4-E2B6-4FD7-8303-E531226A83F3@your.org> In-Reply-To: <20091204202134.GA1716@garage.freebsd.pl> References: <30582AF2-B1E8-4C07-A487-C220845963D2@your.org> <20091204202134.GA1716@garage.freebsd.pl>
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On Dec 4, 2009, at 2:21 PM, Pawel Jakub Dawidek wrote:
> On Wed, Dec 02, 2009 at 02:55:23PM -0600, Kevin wrote:
>>
>> If the slave is completely idle, "zfs receive" takes a fraction of a
>> second. If the slave has been very busy (lots of read activity, no
>> writes - the slave has everything mounted read only), suddenly "zfs
>> receive" can take 30 seconds or more to complete, the whole time it
>> has the filesystem locked. For example, I'd see:
>>
>
> Read activity is related to the dataset on the slave that is being
> received? Is that right?
>
Correct.
> There are two operations that can suspend you file system this way:
> rollback and receive. The suspend is done by acquiring write lock for
> the given file system where every other operation acquires read lock.
> In the end receive to acquire write lock has to wait for all read
> operations to finish.
>
> I'm not sure how your applications use it, but if files are open for
> short period of time only and then closed, you could do something like
> this:
>
> master# curtime=`date "+%Y%m%d%H%M%S"`
> master# zfs snapshot pool/fs@${curtime}
> master# zfs send -i pool/fs@${oldtime} pool/fs@${curtime} | \
> ssh slave zfs recv pool/fs
> slave# zfs clone pool/fs@${curtime} pool/fs_${curtime}
> slave# ln -fs /pool/fs_${curtime} /pool/usethis
>
> Then point your application to use directory /pool/usethis/ (clone,
> instead of received file system). And clean up clones as you wish.
> Read activity on clones shouldn't affect received file system.
This worked much much better, thank you!
I'm still a little blurry on why "zfs receive" is so so so much slower
(going from ms to >20s) if there's any read activity going on at all,
but this seems to make it not matter to us.
-- Kevin
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