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Nicolas Williams wrote:
<blockquote cite="mid:20100704235600.GI15407@oracle.com" type="cite">
<pre wrap="">On Fri, Jul 02, 2010 at 11:37:52PM -0400, Dmitry Zogin wrote:
</pre>
<blockquote type="cite">
<pre wrap="">Well, the hash trees certainly help to achieve data integrity, but
at the performance cost.
</pre>
</blockquote>
<pre wrap=""><!---->
Merkle hash trees cost more CPU cycles, not more I/O. Indeed, they
result in _less_ I/O in the case of RAID-Zn because there's no need to
read the parity unless the checksum doesn't match. Also, how much CPU
depends on the hash function. And HW could help if this became enough
of a problem for us.
</pre>
<blockquote type="cite">
<pre wrap="">Eventually, the file system becomes fragmented, and moving the data
around implies more random seeks with Merkle hash trees.
</pre>
</blockquote>
<pre wrap=""><!---->
Yes, fragmentation is a problem for COW, but that has nothing to do with
Merkle trees. But practically every modern filesystem coalesces writes
into contiguous writes on disk to reach streaming write perfmormance,
and that, like COW, results in filesystem fragmentation.
</pre>
</blockquote>
What I really mean is the defragmentation issue and not the
fragmentation itself. All file systems becomes fragmented, as it is
unavoidable. But the defragmentation of the file system using hash
trees really becomes a problem.<br>
<blockquote cite="mid:20100704235600.GI15407@oracle.com" type="cite">
<pre wrap="">(Of course, you needn't get fragmentation if you never delete or over
write files. You'll get some fragmentation of meta-data, but that's
much easier to garbage collect since meta-data will amount to much less
on disk than data.)
</pre>
</blockquote>
Well, that is really never happens, unless the file system is
read-only. The files are deleted and created all the time.<br>
<blockquote cite="mid:20100704235600.GI15407@oracle.com" type="cite">
<pre wrap="">
Everything we do involves trade-offs.
</pre>
</blockquote>
Yes, but if the performance drop becomes unacceptable, any gain in the
integrity is miserable.<br>
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