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Re: [Networker] Adequate server hardware to feed 3-4 LTO3 drives
2007-10-08 09:49:35
Fazil Saiyed wrote:
What might be your concern regarding throughput or scsi id contention ? or
is it HBA performance ? We have been running atleast 4 drive\HBA over 2
Gb FC
( LTO2) without any issues for long time.
I'm not talking about whether it works without issue, simply about
optimum performance. If you are limiting the speed of your drives by
putting more drives on a SAN connection, then you are not getting the
best performance available from each drive.
Unless your backups are mostly
over the SAN, i doubt that you could utilize the full drive potential with
ethernet.
I agree. However, for a time-critical backup of SAN connected disk
storage, it is important not to overload a SAN connection with too many
drives. Conversely, if you have a situation where your backups are
constrained by network bandwidth, then your drives are not running at
full speed, so you could say that you have too many drives for your
requirement.
Which hardware ( servers) are out there which you would recommend & will
support 20+ HBA to allow one Drive per card ? which is what we have
running between Physical Tape Lib ( Adic i2K & Netapp VTL).
I cannot speak about your particular environment, but I can give you a
real example. I have a storage node with 6 LTO-2 drives connected to 4
HBAs. This backs up a database which is split off as a BCV and mounted
to the storage node. The disks are via a 2Gb switch to a DMX array, and
the backup is of the raw devices, 3 sessions per tape drive.
The total size of the data backed up daily for this database is around
18TB, and usually takes about 23 hours, including all tape labelling and
mounting. This means that each tape drive needs to sustain an average
write speed well in excess of 37 MB/s, with peaks higher than this. If
you assume that the realistic throughput of a 2Gb SAN port is around
150MB/s, then you can see that you would probably not see much
degradation in speed with 3 drives on a 2Gb port, but adding a 4th drive
would cause all the drives to slow down slightly. Any more than 4 would
cause a noticeable reduction in throughput per drive.
My tests on a single LTO-4 drive on a 2Gb switch using bigasm gave a
throughput of 150MB/s, which is about the limit of the SAN connection. I
don't know whether or not the drive can go any faster, since I don't
have a 4Gb SAN to test it with. Extrapolating this would mean a maximum
of two LTO-4 drives on a 4Gb SAN connection, but it is possible that it
might run faster with only one drive on the port, hence my original
statement.
The storage node in question is due to be upgraded from 6 LTO-2 drives
to 4 LTO-4 drives, one drive per HBA. I am hoping for a significant
performance improvement.
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