[Veritas-bu] Backups slow to a crawl

Jeff McCombs jeffm at nicusa.com
Fri Mar 25 09:41:48 CST 2005


    Ok. So I took Darren's suggestion and 'downed' the drive in NBU, drove
out to our facility with a new, unused tape and slapped it into the drive.

I hoped over to my home directory where I've got a good 5G or so of data
with a good mix of file sizes and types and ran the following;

Tar cf - . | compress | dd obs=1024k of=/dev/rmt/1 con=sync

And watched the output of iostat -xtcn, with samples being taken every

And everything looked good for the first, oh.. 5 minutes or so. But the
longer that the stream to tape ran, the worse the performance started to
get. After 5 minutes I began to see the busy:kw/s ratio drop. Busy went from
4-10 % and kw/s 3 MB/Sec when things were good, to 90-100% and kw/s of
100-200k/sec. The longer it ran, the worse it got. Eventually, 6 out of 10
samples were reading 100% busy and a kw/s of 0. The other 4 samples would
range from busy @ 89 - 99, kw/s down into the sub-50k/sec range.

I also checked the output of 'iostat -xtcne' during this run, and while
there were soft and hard errors in the counters, these never actually
increased. 'iostat -nE' provided the following:

rmt/0           Soft Errors: 18 Hard Errors: 0 Transport Errors: 0
Vendor: QUANTUM  Product: DLT8000          Revision: 0250 Serial No: ?P
rmt/1           Soft Errors: 56 Hard Errors: 2 Transport Errors: 2
Vendor: QUANTUM  Product: DLT8000          Revision: 0250 Serial No: ?P

Again though, after performing more tests, I couldn't get these counters to

I did get a response from Veritas. The tech on the phone suggested I muck
with the buffers. Per his instructions, I set NET_BUFFER_SZ to 131072,

I ran a full backup of our system dedicated to managing Checkpoint firewalls
(Sun V100, approx 8GB of data, 100 MB FDX network on the same 3750 switch &
VLAN as the backup system), and performance was actually worse on the first
drive! Both drives sat at approximately 512k/sec, though busy was into the
4-10% range for the duration of the backup.

Aargh. If this was a windows system, I'd be blaming drivers.. I checked
cables, cleaned and reseated the drives, made sure the SCSI controller card
was seated properly, checked termination.. Guess I'll call Overland and have
them get me a new drive.

Many thanks to those of you who have helped me out already. It's much


On 3/24/05 11:14 AM, "Darren Dunham" <ddunham at taos.com> wrote:
> I didn't reply initially because it appeared that you had fixed it.
> I too would be very suspicious of those iostat figures.  To me the high
> busy alongside very low throughput screams drive problems.  Multiplexing
> shouldn't be affecting that.
> If at all possible, I'd try to replicate the error by doing some drive
> testing outside of NBU.
> Down the drive, load a scratch tape, then get busy with 'dd' or
> something.  Can you make it behave similarly?  If so, I'd make it my
> number one suspect.

Jeff McCombs                 |                                    NIC, Inc
Systems Administrator        |                       http://www.nicusa.com
jeffm at nicusa.com             |                                NASDAQ: EGOV
Phone: (703) 909-3277        |        "NIC - the People Behind eGovernment"
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