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From Ashod Nakashian <ashodnakash...@yahoo.com>
Subject Re: Compressed Pristines (Summary)
Date Wed, 04 Apr 2012 18:41:43 GMT




----- Original Message -----
> From: Julian Foad <julianfoad@btopenworld.com>
> To: Ashod Nakashian <ashodnakashian@yahoo.com>
> Cc: Daniel Shahaf <danielsh@elego.de>; Markus Schaber <m.schaber@3s-software.com>;
"mtherieau@gmail.com" <mtherieau@gmail.com>; Subversion Development <dev@subversion.apache.org>
> Sent: Wednesday, April 4, 2012 9:45 PM
> Subject: Re: Compressed Pristines (Summary)
> 
> Ashod Nakashian wrote:
> 
>>  Daniel Shahaf wrote:
>>>>>  Would you like to assess the feasibility of compressing the 
> pristine
>>>>   >store by re-mounting the "pristines" subdirectory as 
> a compressed
>>>>   >subtree in the operating system's file system?
>>>> 
>>>>   No :-)
>>>> 
>>>>   There are two ways to answer this interesting proposition of
>>>>   compressed file-systems. The obvious one is that it isn't 
> something
>>>>   SVN can or should control. The file-system and certainly system
>>>>   drivers are up to the user and any requirement or suggestion of
>>>>   tempering with them is decidedly unwarranted and unexpected from
>>>>   a VCS.
>>> 
>>>  The suggestion wasn't that you teach svn how to change the OS's 
> fs
>>>  settinsg, it was that you investigate how solutions at the OS level
>>>  compare to the other approaches already suggested (custom format -based
>>>  and sqlite-based pristine store).
> 
> Yes, Daniel, that's exactly what I meant.  Sorry, Ash, for not being clear 
> enough.
> 
> Of course that's inherently a non-portable solution that may not be 
> available on all platforms and certainly will differ.  But that's 
> somewhat analogous to support for password storage: in Windows land 
> there's a standard way to do it, in Unixy lands you have to choose your 
> favourite third-party subsystem and externally configure Gnome-keyring or 
> KDE-wallet or whatever you chose.  Of course there are other 
> disadvantages too, I just think it would be interesting to compare the 
> disadvantages with the advantages.
> 
>>  That's an easy question. The answer is that at *best* they'll do as 
> good 
>>  as in-place compression. However, in practice they'll do much worse.
> 
> Ash, what's your measure of "best" here -- just the compression 
> ratio?  What about other kinds of goodness such as the automatic recovering of 
> free space, and the ability to present a plain-text view of the file content 
> through the standard filesystem API which eliminates the need for us to 
> implement a plain-text cache in Subversion?
> 
>>  The 
>>  reason is that the OS level compression works on not only the single file 
> level, 
>>  but actually at the block level. This is to make modifications reasonably 
> fast 
>>  (read compressed data, uncompress, modify, write recompressed data). If the 
> 
>>  complete file is compressed then even changing a single byte (neglecting 
> that no 
>>  storage works on the byte-level anyway) will yield performance that will at 
> 
>>  least linearly degrade by the filesize.
> 
> Changing a single byte is an irrelevant scenario, because pristine files are 
> always created in full and then never modified.

But the OS compression doesn't know that! It's design assumes average file modification operations,
and that's rarely replacing the full file (which is the norm for us). I was only explaining
and justifying the design decision of OS level compression.

> 
>>>  In short: if 'mount -o compress=yes' provides 90% space savings 
> then 
>>  we
>>>  would have little reason to implement space-saving solutions in svn 
> itself.
>>>  But it's the user's, not svn's, responsibility to run that.
> 
> By the way, Daniel, I'm not ruling out the possibility that we may want to 
> provide some glue logic so that Subversion can help the user to set up the 
> feature.  Without such assistance, only expert users would ever benefit.
> 
> It's clearly an interesting topic!
> 
> - Julian
> 

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