dan <dandenson AT gmail DOT com> wrote on 01/10/2009 11:50:57 PM:
> In reply to tmassey's comments. IF you do NOT use compression your
> RAM requirements shrink significantly along with your CPU needs.
> Remember, BackupPC is all about I/O. If you take the one thing that
> really needs a fast cpu away (compression) you are only I/O bound.
I have a philosophical problem with compression and backups (my goal is to
have my backups as simple as possible), so I've never used compression. So
take what I'm going to say with a grain of salt. But:
My understanding of the zlib (gzip) algorithm is that it requires a fixed
(and small) amount of memory for both compression and decompression.
Compression requires more memory than decompression, but both of them are
trivial amounts by today's standards (measured in *kilobytes*). Therefore,
compression should not affect memory requirements in a measurable way.
(Reference: http://www.gzip.org/zlib/zlib_tech.html)
CPU, of course, is obviously needed, but I mentioned that in the original
mail.
In short, the only thing that requires RAM is the file list, whether
compression is used or not. Like I said, I've done servers with ~250GB of
data and half a million files with 512MB of RAM, of which more than 100MB
was still being used as cache, and exactly zero bytes of swap.
So unless you're going to have servers with literally millions of files
(e.g. mail or Usenet servers that store each message as a separate file),
512MB of RAM is plenty. It's not like disk cache is going to help you on
your backup server: it's merely streaming data to disk. 100MB of disk
cache is *plenty* to hold the metadata it'll need...
Tim Massey
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