> Von: ADSM: Dist Stor Manager [mailto:ADSM-L AT VM.MARIST DOT EDU]Im Auftrag
> von
> Bill Boyer
> Gesendet: Donnerstag, 29. April 2004 13:21
> An: ADSM-L AT VM.MARIST DOT EDU
> Betreff: Disk and Tape access over the same fibre adapter
>
>
> Just got a new client that wants to do some LANfree backups, but
> they don't
> believe me when I tell them that the recommended way is to have a separate
> HBA for disk and tape. They don't think that mixing the 2 will be a
> performance issue. I remember a couple years ago this was an issue of data
> corruption, but that has been addresses.
>
> Can someone point me to any whitepapers or documentation
> explaining why they
> should spend a couple more $'s and get an additional adapter for those
> server they want to do LANfree backups on?
>
> TIA,
>
> Bill Boyer
> "Experience is a comb that nature gives us after we go bald." - ??
Hi,
don't know anymore where i got this from. I had this discussion some time
ago and found the following statement:
Mixing disk and tape on a single SCSI bus was rarely done. The workloads are
different, the SCSI profiles are different, and it rarely worked
satisfactorily. In some instances, tape drives would use large blocks and
?data streaming? to maximize performance, tying up the SCSI bus for long
periods of time, while disk drives with smaller block sizes appropriate for
random I/O would get less than their fair share of the SCSI bus. In other
instances, the tape drives would have trouble getting access to the bus
because disk drives would respond faster. It was generally accepted that you
kept disk and tape on separate SCSI buses.
With Fibre Channel, it is possible - in principle - to do better. I/O can be
multiplexed and small blocks can be interleaved in the middle of large
blocks. There is no shared bus to keep a slow device from sending whenever
it is ready. So it is certainly possible with Fibre Channel to have both
disk and tape sending or receiving with little interference. There is always
the issue of having too much data for the bandwidth available, but this is
independent of whether device types are mixed.
Unfortunately, to actually take advantage of these capabilities in Fibre
Channel, an HBA driver would have to be carefully written to handle this.
Sophisticated multithreading would be needed to interleave the I/O fairly.
Furthermore, older SCSI drivers were written using a single SCSI profile for
the whole adapter. This was fine since disk and tape were rarely, if ever,
mixed, so a single profile (one for disk or one for tape) could be used for
the whole adapter and work fine. Often this is not under the user?s
control - the driver picks a profile based on what devices it sees. HBA
drivers could certainly be written to allow different profiles to be used
for different devices at the same time on a single adapter, but this is not
yet a common practice. Thus, if you do mix disk and tape on a single HBA,
you end up either using a disk profile for both disk and tape, or a tape for
both disk and tape. Some devices will be using a non-optimal profile.
Finally, subtle problems have been discovered whose source has been narrowed
down to interference between disk and tape traffic. Typically these problems
occur intermittently, but when they do, devices can go offline leading to
interruption of service.
Also for Performance reason you may consider to have more than one HBA and
does not share Disk and Tape I/O on the same HBA. During Backup your server
has to read the Data from Disk and write it to the Tape Drive. If it's a LTO
2 Drive you may write more than 35 MB/sec to the Tape Drive. This means you
have also to read 35 MB/sec. If those both streams goes over HBA, then this
HBA have to handle at least 70 MB/sec. Considering a 1 Gbit HBA you already
reached the bandwidth of this HBA.
As long as there is no other Disk I/O activity during this backup it may
work without any problems. But, if during the Backup still applications
running which are accessing the Disk, then you may see impact of Backup
performance and impact of the applications performance.
For Backupserver, especially TSM Server, where a high I/O load exist you
should run with several HBAs and separate the disk and tape IO.
For all of these reasons, IBM?s official stance on mixing disk and tape on a
single HBA is a little different from other configurations. IBM will support
mixing disk and tape on a HBA, and the IBM Support Center will accept
problems reported on these configurations. However, if the problem
determination process reveals that the problem in question is caused by the
mixing of the traffic, IBM may choose to tell the customer that the only fix
is to separate the traffic. This normally means installing additional
HBA(s), which can be problematic if a server has insufficient adapter slots.
Over time, more will be learned about which combinations seem to work
satisfactorily, and which combinations have specific problems that will
require separation of the traffic.
In essence, the customer is assuming some risk in these configurations, and
it is strongly recommended to avoid mixing disk and tape traffic on a single
HBA whenever possible.
// Franco Chiellino
// mailto:techi AT chiellino DOT ch
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