Hello Will,
Monday, April 13, 2009, 6:44:47 PM, you wrote:
WM> On Mon, Apr 13, 2009 at 07:03, Robert Milkowski <mi...@task.gda.pl> wrote:
>> Hello Daniel,
>>
>> Thursday, April 9, 2009, 3:35:07 PM, you wrote:
>>
>> DR> Jonathan schrieb:
>>>> OpenSolaris Forums wrote:
>>>>> if you have a snapshot of your files and rsync the same files again,
>>>>> you need to use "--inplace" rsync option , otherwise completely new
>>>>> blocks will be allocated for the new files. that`s because rsync will
>>>>> write entirely new file and rename it over the old one.
>>>>
>>>> ZFS will allocate new blocks either way
>>
>> DR> No it won't. --inplace doesn't rewrite blocks identical on source and
>> DR> target but only blocks which have been changed.
>>
>> Yes, it will. Inplace in rsync has nothing to do with how ZFS works.
WM> But it has big consequences on how rsync uses the file system, and
WM> thus big consequences on how ZFS behaves. --inplace means rsync walks
WM> through the file on the receiving end until it finds a mismatch, and
WM> only then does it write new blocks to disk.
agree.
btw: what I meant by "Yes, it will" is that everytime rsync modifies
any part of file zfs will allocate new fs block for those modification
- regardless if inplace was used or not. However inplace will be more
effective on zfs+snapshots as making a new full copy of a file will be avoided.
>> Now with inplce you're telling rsync to overwrite any changed blocks
>> directly over the original file instead of making a full copy of a
>> file. Everytime you overwrite some data zfs will allocate new blocks
>> only for those blocks and keep the original blocks as long as they are
>> referenced by at least one snapshot.
WM> Exactly. But the consequence of this is that with no --inplace,
WM> rsync+snapshots balloon space usage under ZFS, and with --inplace you
WM> don't get that behavior.
But it's not specific to zfs - will happen one way or
another if you use any file system with snapshots (unless you do have
a dedup buil-in fs).
Nevertheless of course I agree that inplace makes sense if updating
relatively small portions of files.
--
Best regards,
Robert Milkowski
http://milek.blogspot.com
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