Date: Sat, 6 Nov 1999 11:08:58 -0600 (CST) From: Jonathan Lemon <jlemon@americantv.com> To: gurney_j@resnet.uoregon.edu, hackers@freebsd.org Subject: Re: writing much slower than reading... Message-ID: <199911061708.LAA25924@free.pcs> In-Reply-To: <local.mail.freebsd-hackers/19991106013045.13836@hydrogen.fircrest.net>
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In article <local.mail.freebsd-hackers/19991106013045.13836@hydrogen.fircrest.net> you write:
>well, I am working on writing a capture program to do 640x480x12bpp@30fps
>to a raw disk, but writing to the raw device is SOOO slow... the reason
>I say it's slow is the fact that it takes 8 times the system time writing
>than reading...
>
>a bit about the system... k6/2-250, 100mhz system bus, pc100 64meg dimm,
>VIA MVP3 chipset (IDE DMA enabled), IBM-DPTA-372730 hard disk, Hauppauge
>WinCast/TV Model 61351 B226, 3.3-RELEASE...
>
>now the hard disk can push and pull around 20meg/sec w/o any problems..
>but when I time the disk I get:
>$ time dd if=/dev/rwd0s1g of=/dev/null bs=64k count=2048
>2048+0 records in
>2048+0 records out
>134217728 bytes transferred in 5.747521 secs (23352281 bytes/sec)
> 5.75 real 0.01 user 0.21 sys
>$ time dd if=/dev/zero of=/dev/rwd0s1g bs=64k count=2048
>2048+0 records in
>2048+0 records out
>134217728 bytes transferred in 6.281820 secs (21366057 bytes/sec)
> 6.28 real 0.00 user 1.68 sys
Try doing `iostat 1' while doing the transfers. This is what I see here:
tty da0 da1 da2 cpu
tin tout KB/t tps MB/s KB/t tps MB/s KB/t tps MB/s us ni sy in id
TEST A: 0.00 0 0.00 64.00 119 7.47 0.00 0 0.00 0 0 3 0 97
TEST B: 0.00 0 0.00 64.00 419 26.18 0.00 0 0.00 0 0 1 0 99
TEST C: 0.00 0 0.00 64.00 95 5.94 64.00 95 5.94 0 0 1 0 99
TEST D: 63.84 338 21.05 64.00 343 21.41 0.00 0 0.00 0 0 26 2 72
TEST A: (writing) dd if=/dev/zero of=/dev/rda1 bs=1m count=400
TEST B: (reading) dd if=/dev/rda1 of=/dev/zero bs=1m count=400
TEST C: (read & write) dd if=/dev/rda1 of=/dev/rda2 bs=1m count=400
TEST D: (read raw & write fs) dd if=/dev/rda1 of=trash bs=1m count=400
System is a Dell Poweredge 4300 PIII/600, all disks are Seagate ST39103LC.
Disk da0 (where the trash file is written) has softupdates enabled. This
is on a week old -current.
I'm suprised by the last line. Writing to the filesystem is faster
than writing to the raw device? I must be missing something here.
--
Jonathan
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