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RE: pack revprops shards

From: Bolstridge, Andrew <andy.bolstridge_at_intergraph.com>
Date: Mon, 27 Apr 2009 09:39:26 +0100

> -----Original Message-----
> From: Osvaldo Pinali Doederlein [mailto:osvaldo_at_visionnaire.com.br]
> Sent: Friday, April 24, 2009 8:35 PM
> To: users_at_subversion.tigris.org
> Subject: RFE: pack revprops shards
>
> I started this RFE in the Subversion blog: "Packing of the
/db/revprops
> shards. These are still accumulating hundreds of thousands of TINY
> files
> (avg 150 bytes) in my poor Windows server (NTFS really doesn't like
> small files)... with packing, each of these 1000 prop files would be
> replaced by a single ~150Kb blob."
>
> Answer from Hyrum Wright: "Revprops are mutable, and as such their
size
> may change. Modifying a packed revprop would cause the entire shard to
> be rewritten, not just the modified value. Aside from the performance
> issues, this also causes race conditions when multiple revprops are
> being edited at the same time. All of these concerns mean that packing
> of revprops probably won't happen any time soon. What might happen is
a
> migration of revprops to a better storage mechanism, such as sqlite,
> though there are no current plans for that."
>

The answer to that is to load the shard as a memory-mapped file. Then
you update the only appropriate revprop. If you're concerned about
inserting data into the middle of a shard, when packing revprops leave a
chunk of space after each one. Then you can write into the gap. If the
amount of data would overflow the gap, then you'd have to fall back to a
full rewrite of the entire shard.

The only question now is - would packing revprops increase performance
much? I guess they do get read a lot for operations like log, list etc.
I'd say they get read a lot more than past revisions do, so the
performance increase might (*might*) be noticeable.

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Received on 2009-04-27 10:40:26 CEST

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