The algorithm is in S3/FileLists.py compare_filelists().
Check if one side has a file (by name) the other doesn't. If so, there's
nothing to compare.
Check that both files have the same size (as reported by stat() for local
files, and in the remote directory listing)
If checking MD5:
calculate or get the MD5, compare the two values
Date isn't actually compared, because what's in the XML returned by the
object listing from S3 doesn't contain the file date, only the
"Last-Modified" header, which is when the file was uploaded to S3.
Date (really, ctime, mtime, atime) as obtained from local files, when
--preserve is used (the default) _is_ stored in the x-amz-meta-s3cmd-attrs
metadata value for an object. But getting this value back from S3 requires
doing a HEAD on the object itself, for every object, which is really
expensive. So, we don't do that, unless we are comparing MD5s, and the MD5
(really, ETag) value returned in the directory listing indicates the file
was uploaded using multipart upload, in which case the ETag value isn't
the MD5 value for the whole file, but only for the last chunk of the file
committed to disk (not necessarily even the MD5 of the last chunk of the
file). We don't, in general, get that value.
Now, if we are syncing from remote to local, we get x-amz-meta-s3cmd-attrs
value "for free" as a header when we do the GET to get the object, so we do
use it to set the values back to what were originally stored there.
So, the manpage is correct, date is not used in the comparison for syncing
purposes. One could argue that the expensive HEAD call is still cheaper
than calculating the local MD5 of a file, but we can mitigate the local
expense using the --cache-file mechanism such that we only read the local
file once and then read its md5 out of the cache until it changes, so the
HEAD isn't cheaper in general.
To detect a change to a file whose size hasn't changed, but its content
has, we have to do the HEAD call, and calculate the MD5 of the local file
(and use --cache-file to record that for posterity), and compare.
On Sun, Apr 13, 2014 at 5:08 PM, WagnerOne <wag...@wagnerone.com> wrote:
> Hi,
>
> The man page states the following:
>
> --no-check-md5
> Do not check MD5 sums when comparing files for [sync]. Only size will
> be compared. May significantly speed up transfer but may also miss some
> changed files.
>
> When this says "only size will be compared", I'm taking it to mean only
> "size of size and md5" will be compared?
>
> Date (and name, of course), is still used in addition to size if
> --no-check-md5 is passed?
>
> Thanks,
> Mike
>
> --
> wag...@wagnerone.com
> "I have no complaints, ever, about anything."-"Steve McQueen
>
>
>
>
> ------------------------------------------------------------------------------
> Put Bad Developers to Shame
> Dominate Development with Jenkins Continuous Integration
> Continuously Automate Build, Test & Deployment
> Start a new project now. Try Jenkins in the cloud.
> http://p.sf.net/sfu/13600_Cloudbees
> _______________________________________________
> S3tools-general mailing list
> S3tools-general@lists.sourceforge.net
> https://lists.sourceforge.net/lists/listinfo/s3tools-general
>
>
------------------------------------------------------------------------------
Learn Graph Databases - Download FREE O'Reilly Book
"Graph Databases" is the definitive new guide to graph databases and their
applications. Written by three acclaimed leaders in the field,
this first edition is now available. Download your free book today!
http://p.sf.net/sfu/NeoTech
_______________________________________________
S3tools-general mailing list
S3tools-general@lists.sourceforge.net
https://lists.sourceforge.net/lists/listinfo/s3tools-general
