<div dir="ltr">Kevin:<div><br></div><div><div>On Tue, Mar 11, 2014 at 6:18 PM, Kevin Korb <<a href="mailto:kmk@sanitarium.net">kmk@sanitarium.net</a>> wrote:</div><div>>- --checksum should not be used during normal rsync operations. It is</div>
<div>>for special cases only.</div><div><br></div><div>I noted in a reply that we're in that "special case" arena. The use</div><div>case is one where operations are being replicated between systems and</div>
<div>the data files created are meant to be identical. However, bad things</div><div>happen and then repairs are needed. Since it is the operations that</div><div>are being replicated and not the data the time stamps do not match</div>
<div>but the byte-counts should. Unfortunately, the byte-counts don't</div><div>tell the real story and therefore the checksums must be computed.</div><div><br></div><div>There are a lot of products that are doing this type of replication</div>
<div>these days: Subversion's svnsync and (the company I work for)</div><div>WANdisco's SVN MultiSite products to mention just two. There are a</div><div>lot more.</div><div><br></div><div>>Rsync can still have a lot of overhead getting the timestamps via</div>
<div>>stat() but that can't really be helped.</div><div><br></div><div>The overhead of getting the timestamps is tiny compared with the I/O</div><div>computing the checksums.</div><div><br></div><div>>I don't really understand how file mtimes would be cached. How would</div>
<div>>rsync know what mtimes don't match the cache without checking</div><div>>stat()ing the files and then the job is already done so the cache</div><div>>wouldn't accomplish anything.</div><div><br></div>
<div>The concept is that, without file system corruption or intentional</div><div>covert corruption, the tuple {filePath, 64-bit mtime, checksum} is</div><div>an invariant. Cache invalidation is simple: if the mtime is not</div>
<div>exactly the same then re-compute the checksum. Using such a cache</div><div>means that the comparison operation is of the same order of</div><div>magnitude as a "find" through the subtree of the file system versus </div>
<div>reading every file in that subtree AND doing the necessary CPU</div><div>intensive checksum computation. It would be a huge win.</div><div><br></div><div>Thank you.</div><div><br></div><div>Doug</div><div><br></div>
<div>>On 03/11/2014 06:11 PM, Doug Robinson wrote:</div><div>>> Folks:</div><div>>></div><div>>> When using rsync to copy huge amounts of data I've found that a</div><div>>> significant amount of time is spent computing the checksums.</div>
<div>>> Sometimes hours, ... sometimes days - it depends on the total</div><div>>> amount of data checked! And after that sometimes it's only a few</div><div>>> files that need to be updated.</div><div>
>></div><div>>> I've pulled the latest git (rsync-3.1.1pre1) and didn't see</div><div>>> anything to address this (or I missed it?).</div><div>>></div><div>>> I was wondering what folks thought of a proposal to enhance rsync</div>
<div>>> to be able to create and maintain a cache of {filePath, 64-bit</div><div>>> mtime, checksum} beforehand on both source and target systems and</div><div>>> then use that cache later on when asked to sync the two systems</div>
<div>>> together? Then cache entry validation would be a quick stat64() to</div><div>>> make sure that the 64-bit mtime didn't change before sending the</div><div>>> checksum over the wire for comparison.</div>
<div>>></div><div>>> Clearly the cache would need to be completely invalidated (or</div><div>>> re-created) if the file system became corrupt. That could be</div><div>>> handled via an "rm -rf" of the cache.</div>
<div>>></div><div>>> Thoughts?</div><div>>></div><div>>> Thank you.</div><div>>></div><div>>> Doug -- WANdisco // /Non-Stop Data/</div><div>>></div><div>>> t. 925-396-1125 e. <a href="mailto:doug.robinson@wandisco.com">doug.robinson@wandisco.com</a></div>
<div>>> <mailto:<a href="mailto:doug.robinson@wandisco.com">doug.robinson@wandisco.com</a>></div><div>>- --</div><div>>~*-,._.,-*~'`^`'~*-,._.,-*~'`^`'~*-,._.,-*~'`^`'~*-,._.,-*~'`^`'~*-,._.,-*~</div>
<div>> Kevin Korb Phone: (407) 252-6853</div><div>> Systems Administrator Internet:</div><div>> FutureQuest, Inc. Kevin@FutureQuest.net (work)</div>
<div>> Orlando, Florida <a href="mailto:kmk@sanitarium.net">kmk@sanitarium.net</a> (personal)</div><div>> Web page: <a href="http://www.sanitarium.net/">http://www.sanitarium.net/</a></div>
<div>> PGP public key available on web site.</div><div>>~*-,._.,-*~'`^`'~*-,._.,-*~'`^`'~*-,._.,-*~'`^`'~*-,._.,-*~'`^`'~*-,._.,-*~</div><div><br></div><div class="gmail_extra">
<div>-- <br></div><div dir="ltr"><div>Doug Robinson</div><div><div style="color:rgb(80,0,80)"><div dir="ltr" style="color:rgb(34,34,34)"><font color="#666666"><br>WANdisco</font> <font color="#ff9900">//</font> <i><font color="#666666">Non-Stop Data</font></i></div>
</div><div><div dir="ltr"></div></div></div><div><div><a value="+447966783417" style="color:rgb(17,85,204)"><br></a></div><div><font color="#cccccc">t.</font><font color="#999999"> </font><a value="+447966783417" style="color:rgb(17,85,204)">925-396-1125</a></div>
<div><font color="#cccccc">e. </font><a href="mailto:doug.robinson@wandisco.com" style="font-size:small;color:rgb(17,85,204)" target="_blank">doug.robinson@wandisco.com</a><br></div></div></div>
</div></div></div>
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