[asterisk-users] New system for recording - SCSI, SAS or SATA?

[Benny Amorsen]

tony at softins.clara.co.uk (Tony Mountifield) writes:

> I'm in the process of specifying the hardware for some new Asterisk
> systems which will be running a substantial number of conferences
> with recording.
>
> I was wondering what there is to choose between SCSI, SAS and SATA
> disks, in terms of performance for this kind of application.

Modern SCSI, SAS, or SATA drives don't perform differently because of
the interface type. You can't get 15kRPM SATA drives because the market
for those is too small though.

If you record 1 channel in Alaw, you need 2 x 64kbps disk bandwidth, or
16kB/s. If you record 1000 channels, you need 16MB/s from your disks,
which should be easily achievable with even the cheapest disks. However,
that depends on doing sequential writes. You can only do (best case) 120
random writes pr second on a 7200RPM disk without write cache, and you
can reach that limit with just 2 channels, if you have to do a seek pr
packet. The solution there is write cache; 1 second gives you 120
channels and 5 seconds bring you up to 600 channels.

If you are unlucky and the files are placed widely spaced on the drives,
the performance will be lower than those numbers.

So, to get decent performance from many streams, you need a lot of disk
write cache, either on the disk itself (with the risk that a power failure
destroys data), on the controller, or in memory. You can gain a factor
of 2 by going to 15kRPM disks, and another factor of two by doubling the
number of spindles (if you get the layout right). The Linux write cache
can be tweaked for this purpose, but again you risk that a power failure
destroys data.


/Benny