On 17/08/09, Chris Hoogendyk (hoogendyk AT bio.umass DOT edu) wrote:
> Cyrille Bollu wrote:
> >Chris Hoogendyk wrote:
> >> I wouldn't put the holding disks in raid.
<snip>
> http://blogs.zdnet.com/Ou/?p=484.
> 
> While this guy is looking at things like database servers and
> exchange, we ought to be able to interpret this for Amanda. A couple
> of points to note for Amanda: Amanda will use all the holding disk
> drives while doing parallel backups and storing output on the
> holding disks. When it is writing to tape, it is constrained by the
> sequential nature of the tape, and will only be doing one DLE at a
> time from those that it has completed on the holding disks. Also,
> Amanda's access is heavily sequential, although it may have multiple
> parallel processes hitting the drives.

A slow RAID1 off two 7200 RPM SATA disks on a BBU-backed LSI hardware
raid controller can do about 62031 KiB/s write and 86399 KiB/s read.
Those sorts of numbers improve steadily the number of spindles you add
to a RAID collection and the higher the RAID number and (in the case of
writing) if cacheing is enabled.

On 16/08/09, Rory Campbell-Lange (rory AT campbell-lange DOT net) wrote:
> On 14/08/09, Frank Smith (fsmith AT hoovers DOT com) wrote:
> > Chris Hoogendyk wrote:
> With reference to Chris Hoogendyk's email "clarification on
> parallelism", I am very curious to learn if Amanda "...still require[s]
> a DLE to be completed to holding disk before it will send any of it to
> tape..." In our case this is a particularly important question as,
> although we can add in more AoE storage for a DLE, this will only run at
> the speeds above. Do we need a 1TB SAS disk array too?

>From the discussion here it seems preferable to have a DLE on two major
counts. One is that compression can happen prior to writing to tape,
which could result in shoe-shining, and another is that Amanda will be
clearer about the amount of data it will be trying to write to a tape,
in other words it will do a better fit of data to tape.

The most important question I now have to ask is:

    How fast can a SAS-based LTO4 drive write to tape?

Regards
Rory


-- 
Rory Campbell-Lange
Director
rory AT campbell-lange DOT net

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