From: vikram sachdeva (vikram_dude@softhome.net)
Date: Sat Jun 08 2002 - 06:32:02 EDT
Thanks to Alan Nabeth and Selden E Balll Jr for a quick response.
Both of them suggested to install Tape drive on separate controller to
get good transfer rate .But no one could able to answer the exact way of
find the transfer rate although Alan gave some good suggestion where to
look .
Original Question
=================
> System is ES40 with Tru64Unix V4.0G .Earlier TLZ10 DAT drive
> (internal) was installed on separate SCSI controller int this server.
> For some reasons we have to remove that SCSI controller. So we bought
> an SBB TLZ10 DAT drive which we plugged in Disk Shelve Bay (it is a
> hot pluggable drive like the SBB HDD ,blue in color)In this Bay there
> are One Personality Module ,then In Slot 0 one HDD ,Slot 1 one HDD
> Slot 2 is empty and in Slot 3 we have installed Tape Drive.Rest of
> the slots are empty
> After installing DAT in this bay ,the back up time is almost double
> .For us it takes 18 hrs to finish the backup.
> The Questions are
> 1. Why the backup time is doubled ?
2. Is there any way to reduce the backup time(like moving the tape drive
> to lower slot i.e lower scsi id. or by any other means)
> 3.Is there any utility ( like iostat ) form which we can find out the
> data transfer rate for DAT tape drive.
I am including replies of both
Here is Alan's Reply(Very Informative)
=====================
With the tape and disk sharing the same bus now, they
also have to share the available bandwidth. Before,
each bus could have a transfer in progress at the same
time; one reading from disk, the other writing to tape.
The only way to get back the performance, is to move
the tape back to its own SCSI bus.
The disk drivers are responsible for collecting the data
used by iostat(1). The tape drives don't collect that
data. The drive may count transfers and bytes transferred.
On V5, you may be able to get this data with hwmgr(8). You'd
have to get successive samples over a fixed time and calculate
the data rate by hand, but if you don't have to do it often
it can work.
The tape drive may also keep tranfer counts, probably available
in some SCSI log page. You could query the page at the
beginning
of the backup, again at the end and note the elapsed time. This
will give you a data rate for the whole backup. Depending on
the backup software it may provide the same data.
Here is Selden's Reply
> 1. Why the backup time is doubled ?
There are two possibilities:
1) Because the new drive is having trouble writing to the tapes that you
are using. Error recovery slows things down. Check your system hardware
error logs to see if this is happening. e.g. uerf -R -o full | more
2) Because both the disk and the tape drive are on the same SCSI bus.
As best we can tell, current model tape drives do not actually support
disconnect/reconnect. This causes serious delays for disk accesses. As a
side effect, your tape drive is no longer streaming.
> 2. Is there any way to reduce the backup time(like moving the tape
drive
> to lower slot i.e lower scsi id. or by any other means)
Move it to a different SCSI bus.
Our experience has been that tape drives should be on a SCSI bus
by themselves both for performance and for system reliability.
> 3.Is there any utility (like iostat) form which we can find out the
> data transfer rate for DAT tape drive.
Sorry: I don't know of one. We just dd a file and see how long it takes.
Please be sure to summarize and let us know whatever benchmarks and tape
monitoring programs you learn about.
I hope this helps a little.
Regards
Vikram
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