Date: Mon, 3 Aug 1998 04:35:16 +0200 (MET DST) From: "D. Rock" <rock@cs.uni-sb.de> To: current@FreeBSD.ORG Subject: IO performance (UFS read clustering), bad ZIP drive performance Message-ID: <13765.8590.159445.703954@doom>
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I get unacceptable performance from my SCSI ZIP drive (compared to other
operating systems). While UFS writing is OK (up to the capabilities of the
drive: 600-1000 kB/s), everything else (UFS read, MSDOSFS r/w, mtools)
is extremely slow (down to 80 kB/s).
It seems the SCSI ZIP drive is only able to perform a maximum of 40-45
requests per second.
UFS writes get clustered to larger chunks, resulting in good throughput.
But it seems that reads don't get clustered at all. iostat tells me, it is
doing 40-45 tps, resulting to a throughput of 160 kB/s with 4kB block size.
MSDOSFS is worse with 2kB cluster size. And no read/write clustering at all.
mtools perform best with the raw device (mtools are doing read/write
clustering for themselves). But if I use the block device, I wouldn't
expect the performance go down. But with block devices a single large
request seems to be divided into several small ones (of 2k each).
This can be verified with a simple dd run:
# dd if=/dev/sd0 of=/dev/null bs=512k
and in another window
# iostat 1
tty sd0 cd0 cpu
tin tout sps tps msps sps tps msps us ni sy in id
0 5 11 1 0.0 0 0 0.0 3 95 2 1 0
0 271828 454 0.0 0 0 0.0 0 88 9 4 0
0 271574 385 0.0 0 0 0.0 0 88 10 2 0
0 271778 438 0.0 0 0 0.0 0 84 14 2 0
I have seen this behaviour in 3 different setups, all with different SCSI
controllers: Adaptec 2940UW, NCR810, AIC6360 chip
Why doesn't read clustering on UFS work, but write clustering(*)?
Why does the (now simulated?) block device divide I/O into chunks of only 2k?
Only UFS read performance is of big interest to me (writes are OK). I don't
use MSDOSFS on ZIP drives and can configure mtools to use the raw device.
Daniel
(*) A really old current (July 1997) doesn't have this problem.
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