zfs discuss - Sep 2010 - Algorithm of block size estimation

Hi,

Can anybody help me give the link on the code snippet of block size
estimation?

I want to know when ZFS makes a decision on the block size used for a file.
Does ZFS estimate it based on the length of file when the create event of
file is committed to disk during txg commit?

If so,  is the block size of a file changed during  the lifetime of the
file? I suppose the answer is no.

Thank you for your kindly help!

best regards,
hanzhu
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On 02/09/2010 11:18, Zhu Han wrote:
> Can anybody help me give the link on the code snippet of block size
> estimation?
See the zfs_write() function.

-- 
Darren J Moffat

Thank you!

Here is my understanding, leave it here as reference for others. If
it''s not
correct, please point it out.

ZFS estimates the size of block only when the file only has single block and
it is extended.  This is because dmu_object_set_blocksize() only set the
block size when the object contains a single block.  Once the file is beyond
single block, the block size is fixed and never be shrinked.

ZFS always sets the block size as the power 2 ceil of file size. The largest
block size is specified by the file system property. One special case is if
the current file block size is not power 2, it''s new block size can be
greater than the file system recordsize property. Does anybody know why
there is such a short circuit?

best regards,
hanzhu


On Thu, Sep 2, 2010 at 6:30 PM, Darren J Moffat <darrenm at
opensolaris.org>wrote:
> On 02/09/2010 11:18, Zhu Han wrote:
>
>> Can anybody help me give the link on the code snippet of block size
>> estimation?
>>
>
> See the zfs_write() function.
>
> --
> Darren J Moffat
>
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