Linux 2.4/2.5 SCSI considerably slower than FreeBSD

With a Fujitsu MAM3184 (U160, 15Krpm 18GB) disk and a Tekram DC-390U3W controller (sym53c8xx_2 driver) on lk 2.5.15 I get:

$ time dd if=/dev/sdb of=/dev/null bs=64k count=16k
16384+0 records in
16384+0 records out
real 0m18.948s  user 0m0.010s  sys 0m4.090s  

That is 56.67 MB/sec (MB == 10^6).

$ time sg_dd if=/dev/sg1 of=/dev/null bs=512 count=2m time=1
time to transfer data was 18.786448 secs, 57.16 MB/sec
2097152+0 records in
2097152+0 records out
real 0m18.799s  user 0m0.030s  sys 0m3.010s

$ time sgm_dd if=/dev/sg1 of=/dev/null bs=512 count=2m time=1
time to transfer data was 18.777035 secs, 57.18 MB/sec
2097152+0 records in
2097152+0 records out
real 0m18.781s  user 0m0.020s  sys 0m0.100s

The MAM3184 disk was recently reviewed
( see http://www4.tomshardware.com/storage/02q2/020415/index.html ) and those speeds are very close to the maximum in their benchmarks (and Fujitsu's published specifications) for outer track reads.

I am impressed by dd's performance in the lk 2.5 series. When sg_dd and sgm_dd are used they bypass the block subsystem and issue 64KB SCSI read commands (in this case). As can be seen above, this improves the throughput by about 1 % compared to dd. CPU utilization (on a Athlon 1.2 GHz box with 512 MB of DDR ram) is a little more expensive with dd (4 seconds compared with 3 seconds). The "sgm_dd" command uses mmap() to do "zero copy" reads which is why its CPU utilization is so low.

lower than sg_dd. No doubt FreeBSD would also perform well but I doubt it could beat linux (2.5) by the type of margin your measurements indicate. [For sequential reads, tagged queueing will not have a significant impact.] It is also worth noting that the new aic7xxx and sym53c8xx_2 drivers are essentially the same on Linux and FreeBSD (i.e. same code base, same maintainers).

Using scsi_debug (a ram disk) as a dummy scsi load yields:

# time dd if=/dev/sdc of=/dev/null bs=64k count=2k
2048+0 records in
2048+0 records out
real 0m1.082s  user 0m0.000s  sys 0m0.990s
That's 124 MB/sec and the CPU utilization is dominating. The
"sgm_dd" command yields 850 MB/sec for the same transfer.

Doug Gilbert

In a live system (actually, it's idle, but every 5 s, there is a short burst of disk activity -- reiserfs and ext3fs in use here, something is going on there), sg_dd is not really better than plain dd:

gives: 27,56 MB/s. A little better than dd, but still much less than FreeBSD's.

It does. I trust my last figures. I don't trust the above too much because of the bogus 5 s interval write burst. Not sure where it comes from, ext3fs or reiserfs, I'd pretty much like something like an inverse
strace: "who is writing to my disk?"

I know that, and I assume it's not the fault of the drivers themselves (that's because the "far from theoretical optimum" holds on either hardware).

Looks like either you're lucky or like Linux has some bad VM interaction or Linux has issues with U160-drives on UWSCSI adapters (yours does U160, ours don't) that FreeBSD doesn't have. (FreeBSD's IDE driver is not as good as the Linux 2.5 IDE driver, so in the end, FreeBSD and Linux are even again ;-)

Matthias Andree

hmmm.

# sync; sync; time dd if=/dev/sda of=/dev/null bs=64k count=10000 10000+0 records in
10000+0 records out

real    0m12.809s
user    0m0.020s
sys     0m2.560s

(first read, the data shouldn't have been cached)
-> 48.79 MB/s

I wouldn't suspect there's a problem, the numbers look reasonable.

$ uname -r
2.4.19-pre2

Disk:
Host: scsi0 Channel: 00 Id: 02 Lun: 00
Vendor: SEAGATE Model: ST336706LW Rev: 0109
Type: Direct-Access ANSI SCSI revision: 03
(i.e., 36GiB 160MB/s Seagate Cheetah)

Controller - HP netserver onboard sym53c896.

T.

Matthias,
It is difficult to answer your main question but here are some more figures on a pretty fast disk restricted by a SCSI UW parallel bus (maximum bandwidth: 40 MB/sec).

Disk read speeds (MB/sec) for various block sizes
=================================================

block  ||        dd          |       sg_dd       |       sgm_dd
size   ||  lk2.4  | lk2.5    |  lk2.4  | lk2.5   |  lk2.4  | lk2.5
==================================================================
2 KB   ||   31.5     35.1    |   16.3     15.6   |   17.0     16.2
4 KB   ||   31.4     35.0    |   22.4     22.4   |   23.9     23.8
8 KB   ||   31.3     35.7    |   27.7     27.7   |   29.9     29.8
16 KB  ||   31.4     35.7    |   31.3     31.4   |   34.2     34.2
32 KB  ||   31.4     35.7    |   33.6     33.6   |   36.9     36.9
64 KB  ||   31.5     35.7    |   34.7     34.7   |   38.2     38.2
128 KB ||   31.5     35.5    |   34.9     34.9   |   39.0     39.0
256 KB ||   31.5     35.5    |   33.3     33.4   |   39.3     39.3

Reading a Fujitsu MAM 3184MP disk (SCSI u160 capable) on the Ultra Wide channel (max bandwidth 40 MB/sec) of a Tekram DC-390U3W dual controller. The HBA driver is sym53c8xx_2 (version sym-2.1.17a for lk 2.4.19-pre7 and version sym-2.1.16a for lk 2.5.17).

The block size for the dd command was the figure given to the "bs" argument and the effective figure given to "bpt" for sg_dd and sgm_dd (i.e. bpt=block_size/512). [Dropping the block size for dd to 512 and 256 bytes made virtually no difference either.] sg_dd uses the sg interface to access the disk and copies data via kernel buffers while sgm_dd memory maps those kernel buffers to the user space. In all cases 1 GB of data was read from the outer tracks (lba==0).

Conclusions:
- the block size given to dd has very little impact on its performance
- the dd in lk 2.5 performs better than the one in lk 2.4
- otherwise performance is roughly similar
- obviously the 40 MB/sec bandwidth of the scsi UW bus is the limiting factor with larger block sizes

Here are some numbers for the same disk on the U160 channel of the same DC-390U3W controller in lk 2.5.17:

          dd         sg_dd       sgm_dd
2 KB     56.2        20.5        22.8
8 KB     56.8        50.0        56.7
32 KB    56.7        57.0        57.2

The test system is a 1.2 GHz Athlon on a Asus A7M266 MoBo with 512 MB of DDR ram. The test disk had no fs mounted and was on a bus by itself.

Doug Gilbert
22nd May 2002