Dear all,
Thank you answering my last question in terms of the RAID6 volume
scanning; I have another question.
I built a device with a large number of disks (see below). Each RAID set
/dev/md1[0-2] has 15 drives attached to it....and the
My concern is that ~write~ I/O is really poor; I have tweaked a number
of kernel parameters and my ~read~ I/O is rather good (a fact of 1:3)
i.e. read speed seems to be three times quicker.
I have trolled through Google to see if there are any suggestions that I
can look at, but I wanted some thoughts to see if I can improve things.
Some considerations:
All the RAIDing is done via mdadm (hence my question to this forum).
There are no real proper hardware controllers. I am will never expect
anything other than mediocre / poor performance out of this system, but
the parity between read and write speeds seem high.
System is Centos 5.4 with a 2.6.18-164.15.1.el5 kernel #1 SMP Wed Mar 17
11:30:06 EDT 2010 x86_64 x86_64 x86_64 GNU/Linux
mdadm - v2.6.9 - 10th March 2009
4G RAM
Any obvious pointer gratefully received.
Best Wishes
Max
The Drives:
/dev/md10 18T 2.1T 16T 12% /media/proxies
/dev/md11 18T 5.5T 13T 31% /media/mediastorage1
/dev/md12 18T 3.2T 15T 18% /media/mediastorage2
md: Autodetecting RAID arrays.
md: autorun ...
md: considering sdc1 ...
md: adding sdc1 ...
md: adding sdd1 ...
md: adding sde1 ...
md: adding sdf1 ...
md: adding sdg1 ...
md: adding sdh1 ...
md: adding sdi1 ...
md: adding sdj1 ...
md: adding sdk1 ...
md: adding sdl1 ...
md: adding sdm1 ...
md: adding sdn1 ...
md: adding sdo1 ...
md: adding sdp1 ...
md: adding sdq1 ...
md: sdr1 has different UUID to sdc1
md: sds1 has different UUID to sdc1
md: sdt1 has different UUID to sdc1
md: sdu1 has different UUID to sdc1
md: sdv1 has different UUID to sdc1
md: sdw1 has different UUID to sdc1
md: sdx1 has different UUID to sdc1
md: sdy1 has different UUID to sdc1
md: sdz1 has different UUID to sdc1
md: sdaa1 has different UUID to sdc1
md: sdab1 has different UUID to sdc1
md: sdac1 has different UUID to sdc1
md: sdad1 has different UUID to sdc1
md: sdae1 has different UUID to sdc1
md: sdaf1 has different UUID to sdc1
md: sdag1 has different UUID to sdc1
md: sdah1 has different UUID to sdc1
md: sdai1 has different UUID to sdc1
md: sdaj1 has different UUID to sdc1
md: sdak1 has different UUID to sdc1
md: sdal1 has different UUID to sdc1
md: sdam1 has different UUID to sdc1
md: sdan1 has different UUID to sdc1
md: sdao1 has different UUID to sdc1
md: sdap1 has different UUID to sdc1
md: sdaq1 has different UUID to sdc1
md: sdar1 has different UUID to sdc1
md: sdas1 has different UUID to sdc1
md: sdat1 has different UUID to sdc1
md: sdau1 has different UUID to sdc1
md: created md10
md: bind<sdq1>
md: bind<sdp1>
md: bind<sdo1>
md: bind<sdn1>
md: bind<sdm1>
md: bind<sdl1>
md: bind<sdk1>
md: bind<sdj1>
md: bind<sdi1>
md: bind<sdh1>
md: bind<sdg1>
md: bind<sdf1>
md: bind<sde1>
md: bind<sdd1>
md: bind<sdc1>
md: running:<sdc1><sdd1><sde1><sdf1><sdg1><sdh1><sdi1><sdj1><sdk1><sdl1><sdm1><sdn1><sdo1><sdp1><sdq1>
raid5: automatically using best checksumming function: generic_sse
generic_sse: 8852.000 MB/sec
raid5: using function: generic_sse (8852.000 MB/sec)
raid6: int64x1 1921 MB/s
raid6: int64x2 2355 MB/s
raid6: int64x4 2187 MB/s
raid6: int64x8 1812 MB/s
raid6: sse2x1 4082 MB/s
raid6: sse2x2 4582 MB/s
raid6: sse2x4 6808 MB/s
raid6: using algorithm sse2x4 (6808 MB/s)
md: raid6 personality registered for level 6
md: raid5 personality registered for level 5
md: raid4 personality registered for level 4
raid5: device sdc1 operational as raid disk 0
raid5: device sdd1 operational as raid disk 1
raid5: device sde1 operational as raid disk 2
raid5: device sdf1 operational as raid disk 3
raid5: device sdg1 operational as raid disk 4
raid5: device sdh1 operational as raid disk 5
raid5: device sdi1 operational as raid disk 6
raid5: device sdj1 operational as raid disk 7
raid5: device sdk1 operational as raid disk 8
raid5: device sdl1 operational as raid disk 9
raid5: device sdm1 operational as raid disk 10
raid5: device sdn1 operational as raid disk 11
raid5: device sdo1 operational as raid disk 12
raid5: device sdp1 operational as raid disk 13
raid5: device sdq1 operational as raid disk 14
raid5: allocated 15812kB for md10
raid5: raid level 6 set md10 active with 15 out of 15 devices, algorithm 2
RAID5 conf printout:
--- rd:15 wd:15 fd:0
disk 0, o:1, dev:sdc1
disk 1, o:1, dev:sdd1
disk 2, o:1, dev:sde1
disk 3, o:1, dev:sdf1
disk 4, o:1, dev:sdg1
disk 5, o:1, dev:sdh1
disk 6, o:1, dev:sdi1
disk 7, o:1, dev:sdj1
disk 8, o:1, dev:sdk1
disk 9, o:1, dev:sdl1
disk 10, o:1, dev:sdm1
disk 11, o:1, dev:sdn1
disk 12, o:1, dev:sdo1
disk 13, o:1, dev:sdp1
disk 14, o:1, dev:sdq1
md: considering sdr1 ...
md: adding sdr1 ...
md: adding sds1 ...
md: adding sdt1 ...
md: adding sdu1 ...
md: adding sdv1 ...
md: adding sdw1 ...
md: adding sdx1 ...
md: adding sdy1 ...
md: adding sdz1 ...
md: adding sdaa1 ...
md: adding sdab1 ...
md: adding sdac1 ...
md: adding sdad1 ...
md: adding sdae1 ...
md: adding sdaf1 ...
md: sdag1 has different UUID to sdr1
md: sdah1 has different UUID to sdr1
md: sdai1 has different UUID to sdr1
md: sdaj1 has different UUID to sdr1
md: sdak1 has different UUID to sdr1
md: sdal1 has different UUID to sdr1
md: sdam1 has different UUID to sdr1
md: sdan1 has different UUID to sdr1
md: sdao1 has different UUID to sdr1
md: sdap1 has different UUID to sdr1
md: sdaq1 has different UUID to sdr1
md: sdar1 has different UUID to sdr1
md: sdas1 has different UUID to sdr1
md: sdat1 has different UUID to sdr1
md: sdau1 has different UUID to sdr1
md: created md11
md: bind<sdaf1>
md: bind<sdae1>
md: bind<sdad1>
md: bind<sdac1>
md: bind<sdab1>
md: bind<sdaa1>
md: bind<sdz1>
md: bind<sdy1>
md: bind<sdx1>
md: bind<sdw1>
md: bind<sdv1>
md: bind<sdu1>
md: bind<sdt1>
md: bind<sds1>
md: bind<sdr1>
md: running:<sdr1><sds1><sdt1><sdu1><sdv1><sdw1><sdx1><sdy1><sdz1><sdaa1><sdab1><sdac1><sdad1><sdae1><sdaf1>
raid5: device sdr1 operational as raid disk 0
raid5: device sds1 operational as raid disk 1
raid5: device sdt1 operational as raid disk 2
raid5: device sdu1 operational as raid disk 3
raid5: device sdv1 operational as raid disk 4
raid5: device sdw1 operational as raid disk 5
raid5: device sdx1 operational as raid disk 6
raid5: device sdy1 operational as raid disk 7
raid5: device sdz1 operational as raid disk 8
raid5: device sdaa1 operational as raid disk 9
raid5: device sdab1 operational as raid disk 10
raid5: device sdac1 operational as raid disk 11
raid5: device sdad1 operational as raid disk 12
raid5: device sdae1 operational as raid disk 13
raid5: device sdaf1 operational as raid disk 14
raid5: allocated 15812kB for md11
raid5: raid level 6 set md11 active with 15 out of 15 devices, algorithm 2
RAID5 conf printout:
--- rd:15 wd:15 fd:0
disk 0, o:1, dev:sdr1
disk 1, o:1, dev:sds1
disk 2, o:1, dev:sdt1
disk 3, o:1, dev:sdu1
disk 4, o:1, dev:sdv1
disk 5, o:1, dev:sdw1
disk 6, o:1, dev:sdx1
disk 7, o:1, dev:sdy1
disk 8, o:1, dev:sdz1
disk 9, o:1, dev:sdaa1
disk 10, o:1, dev:sdab1
disk 11, o:1, dev:sdac1
disk 12, o:1, dev:sdad1
disk 13, o:1, dev:sdae1
disk 14, o:1, dev:sdaf1
md: considering sdag1 ...
md: adding sdag1 ...
md: adding sdah1 ...
md: adding sdai1 ...
md: adding sdaj1 ...
md: adding sdak1 ...
md: adding sdal1 ...
md: adding sdam1 ...
md: adding sdan1 ...
md: adding sdao1 ...
md: adding sdap1 ...
md: adding sdaq1 ...
md: adding sdar1 ...
md: adding sdas1 ...
md: adding sdat1 ...
md: adding sdau1 ...
md: created md12
md: bind<sdau1>
md: bind<sdat1>
md: bind<sdas1>
md: bind<sdar1>
md: bind<sdaq1>
md: bind<sdap1>
md: bind<sdao1>
md: bind<sdan1>
md: bind<sdam1>
md: bind<sdal1>
md: bind<sdak1>
md: bind<sdaj1>
md: bind<sdai1>
md: bind<sdah1>
md: bind<sdag1>
md: running:<sdag1><sdah1><sdai1><sdaj1><sdak1><sdal1><sdam1><sdan1><sdao1><sdap1><sdaq1><sdar1><sdas1><sdat1><sdau1>
raid5: device sdag1 operational as raid disk 0
raid5: device sdah1 operational as raid disk 1
raid5: device sdai1 operational as raid disk 2
raid5: device sdaj1 operational as raid disk 3
raid5: device sdak1 operational as raid disk 4
raid5: device sdal1 operational as raid disk 5
raid5: device sdam1 operational as raid disk 6
raid5: device sdan1 operational as raid disk 7
raid5: device sdao1 operational as raid disk 8
raid5: device sdap1 operational as raid disk 9
raid5: device sdaq1 operational as raid disk 10
raid5: device sdar1 operational as raid disk 11
raid5: device sdas1 operational as raid disk 12
raid5: device sdat1 operational as raid disk 13
raid5: device sdau1 operational as raid disk 14
raid5: allocated 15812kB for md12
raid5: raid level 6 set md12 active with 15 out of 15 devices, algorithm 2
RAID5 conf printout:
--- rd:15 wd:15 fd:0
disk 0, o:1, dev:sdag1
disk 1, o:1, dev:sdah1
disk 2, o:1, dev:sdai1
disk 3, o:1, dev:sdaj1
disk 4, o:1, dev:sdak1
disk 5, o:1, dev:sdal1
disk 6, o:1, dev:sdam1
disk 7, o:1, dev:sdan1
disk 8, o:1, dev:sdao1
disk 9, o:1, dev:sdap1
disk 10, o:1, dev:sdaq1
disk 11, o:1, dev:sdar1
disk 12, o:1, dev:sdas1
disk 13, o:1, dev:sdat1
disk 14, o:1, dev:sdau1
md: ... autorun DONE.
device-mapper: multipath: version 1.0.5 loaded
EXT3 FS on dm-0, internal journal
kjournald starting. Commit interval 5 seconds
EXT3 FS on dm-1, internal journal
EXT3-fs: mounted filesystem with ordered data mode.
kjournald starting. Commit interval 5 seconds
EXT3 FS on dm-2, internal journal
EXT3-fs: mounted filesystem with ordered data mode.
kjournald starting. Commit interval 5 seconds
EXT3 FS on dm-3, internal journal
EXT3-fs: mounted filesystem with ordered data mode.
kjournald starting. Commit interval 5 seconds
EXT3 FS on md0, internal journal
EXT3-fs: mounted filesystem with ordered data mode.
SGI XFS with ACLs, security attributes, large block/inode numbers, no debug enabled
SGI XFS Quota Management subsystem
Filesystem "md10": Disabling barriers, trial barrier write failed
XFS mounting filesystem md10
Ending clean XFS mount for filesystem: md10
Filesystem "md11": Disabling barriers, trial barrier write failed
XFS mounting filesystem md11
Ending clean XFS mount for filesystem: md11
Filesystem "md12": Disabling barriers, trial barrier write failed
XFS mounting filesystem md12
Ending clean XFS mount for filesystem: md12
Sample of iostat from one of the RAID arrays. This has been taken
whilst the RAID is being written to.
Device: tps MB_read/s MB_wrtn/s MB_read MB_wrtn
md12 417.16 0.00 3.55 55 3736560
Device: tps MB_read/s MB_wrtn/s MB_read MB_wrtn
md12 1173.50 0.00 10.00 0 19
Device: tps MB_read/s MB_wrtn/s MB_read MB_wrtn
md12 845.50 0.00 7.19 0 14
Device: tps MB_read/s MB_wrtn/s MB_read MB_wrtn
md12 1086.50 0.00 9.25 0 18
Device: tps MB_read/s MB_wrtn/s MB_read MB_wrtn
md12 1117.00 0.00 9.50 0 19
Device: tps MB_read/s MB_wrtn/s MB_read MB_wrtn
md12 148.76 0.00 1.27 0 2
Device: tps MB_read/s MB_wrtn/s MB_read MB_wrtn
md12 8.00 0.00 0.06 0 0
Device: tps MB_read/s MB_wrtn/s MB_read MB_wrtn
md12 190.50 0.00 1.62 0 3
Device: tps MB_read/s MB_wrtn/s MB_read MB_wrtn
md12 0.00 0.00 0.00 0 0
lspci produces
<snip>
07:00.0 RAID bus controller: Silicon Image, Inc. SiI 3124 PCI-X Serial ATA Controller (rev 02)
07:02.0 RAID bus controller: Silicon Image, Inc. SiI 3124 PCI-X Serial ATA Controller (rev 02)
07:04.0 RAID bus controller: Silicon Image, Inc. SiI 3124 PCI-X Serial ATA Controller (rev 02)
mdadm.conf produces
cat /etc/mdadm.conf
# mdadm.conf written out by anaconda
DEVICE partitions
MAILADDR root
ARRAY /dev/md0 level=raid1 num-devices=2 uuid=7d7b19e6:56cc90cc:3cb166bd:b8086f29
ARRAY /dev/md1 level=raid1 num-devices=2 uuid=3782d93d:a491ffd4:f32c1014:94a2b3f7
ARRAY /dev/md10 level=raid6 num-devices=15 uuid=5ca86e2a-3b86-4c0b-9a7a-59143bdcd0f1
ARRAY /dev/md11 level=raid6 num-devices=15 uuid=61188c90-4825-44c5-8fac-9bc82a5799fe
ARRAY /dev/md12 level=raid6 num-devices=15 uuid=fa939816-1d0f-4eaa-98dd-c131449c3921
cat /proc/mdstat reports
cat /proc/mdstat
Personalities : [raid1] [raid6] [raid5] [raid4]
md0 : active raid1 sdb1[1] sda1[0]
1044096 blocks [2/2] [UU]
md10 : active raid6 sdc1[0] sdd1[1] sde1[2] sdf1[3] sdg1[4] sdh1[5] sdi1[6] sdj1[7] sdk1[8] sdl1[9] sdm1[10] sdn1[11] sdo1[12] sdp1[13] sdq1[14]
19046800448 blocks level 6, 64k chunk, algorithm 2 [15/15] [UUUUUUUUUUUUUUU]
md11 : active raid6 sdr1[0] sds1[1] sdt1[2] sdu1[3] sdv1[4] sdw1[5] sdx1[6] sdy1[7] sdz1[8] sdaa1[9] sdab1[10] sdac1[11] sdad1[12] sdae1[13] sdaf1[14]
19046766336 blocks level 6, 256k chunk, algorithm 2 [15/15] [UUUUUUUUUUUUUUU]
md12 : active raid6 sdag1[0] sdah1[1] sdai1[2] sdaj1[3] sdak1[4] sdal1[5] sdam1[6] sdan1[7] sdao1[8] sdap1[9] sdaq1[10] sdar1[11] sdas1[12] sdat1[13] sdau1[14]
19046766336 blocks level 6, 256k chunk, algorithm 2 [15/15] [UUUUUUUUUUUUUUU]
md1 : active raid1 sdb2[1] sda2[0]
77103872 blocks [2/2] [UU]
sysctl.conf reports:
# Kernel sysctl configuration file for Red Hat Linux
#
# For binary values, 0 is disabled, 1 is enabled. See sysctl(8) and
# sysctl.conf(5) for more details.
# Controls IP packet forwarding
net.ipv4.ip_forward = 0
# Controls source route verification
net.ipv4.conf.default.rp_filter = 1
# Do not accept source routing
net.ipv4.conf.default.accept_source_route = 0
# Controls the System Request debugging functionality of the kernel
kernel.sysrq = 0
# Controls whether core dumps will append the PID to the core filename
# Useful for debugging multi-threaded applications
kernel.core_uses_pid = 1
# Controls the use of TCP syncookies
net.ipv4.tcp_syncookies = 1
# Controls the maximum size of a message, in bytes
kernel.msgmnb = 65536
# Controls the default maxmimum size of a mesage queue
kernel.msgmax = 65536
# Controls the maximum shared segment size, in bytes
kernel.shmmax = 68719476736
# Controls the maximum number of shared memory segments, in pages
kernel.shmall = 4294967296
net.core.rmem_max = 16777216
net.core.wmem_max = 16777216
## increase Linux autotuning TCP buffer limits
## min, default, and max number of bytes to use
## set max to at least 4MB, or higher if you use very high BDP paths
net.ipv4.tcp_rmem = 4096 87380 16777216
net.ipv4.tcp_wmem = 4096 65536 16777216
## don't cache ssthresh from previous connection
net.ipv4.tcp_no_metrics_save = 1
net.ipv4.tcp_moderate_rcvbuf = 1
## recommended to increase this for 1000 BT or higher
net.core.netdev_max_backlog = 8000
## for 10 GigE, use this, uncomment below
#net.core.netdev_max_backlog = 30000
## Turn off timestamps if you're on a gigabit or very busy network
## Having it off is one less thing the IP stack needs to work on
net.ipv4.tcp_timestamps = 0
## disable tcp selective acknowledgements.
net.ipv4.tcp_sack = 0
##enable window scaling
net.ipv4.tcp_window_scaling = 1
vm.swappiness = 25
Finally from a boot script in /etc/rc.d/rc.local
echo -n 25000> /proc/sys/dev/raid/speed_limit_min
echo -n 400000> /proc/sys/dev/raid/speed_limit_max
blockdev --setra 16384 /dev/sd[a-z]
blockdev --setra 16384 /dev/sda[a-u]
blockdev --setra 16384 /dev/md1[0-2]
blockdev --setra 16384 /dev/md[0-1]
for driveletter in a b c d e f g h i j k l m n o p q r s t u v w x y z
do
echo -n 1024> /sys/block/sd$driveletter/queue/nr_requests
echo -n 1024> /sys/block/sda$driveletter/queue/nr_requests
done
hdparm has produced the following for me:
/dev/sda:
Timing buffered disk reads: 218 MB in 3.00 seconds = 72.61 MB/sec
/dev/sdaa:
Timing buffered disk reads: 218 MB in 3.01 seconds = 72.45 MB/sec
/dev/sdb:
Timing buffered disk reads: 218 MB in 3.01 seconds = 72.48 MB/sec
/dev/sdab:
Timing buffered disk reads: 198 MB in 3.00 seconds = 66.00 MB/sec
/dev/sdc:
Timing buffered disk reads: 200 MB in 3.00 seconds = 66.65 MB/sec
/dev/sdac:
Timing buffered disk reads: 212 MB in 3.01 seconds = 70.35 MB/sec
/dev/sdd:
Timing buffered disk reads: 198 MB in 3.02 seconds = 65.66 MB/sec
/dev/sdad:
Timing buffered disk reads: 204 MB in 3.01 seconds = 67.74 MB/sec
/dev/sde:
Timing buffered disk reads: 216 MB in 3.00 seconds = 71.93 MB/sec
/dev/sdae:
Timing buffered disk reads: 200 MB in 3.02 seconds = 66.31 MB/sec
/dev/sdf:
Timing buffered disk reads: 200 MB in 3.02 seconds = 66.24 MB/sec
/dev/sdaf:
Timing buffered disk reads: 238 MB in 3.00 seconds = 79.23 MB/sec
/dev/sdg:
Timing buffered disk reads: 198 MB in 3.00 seconds = 66.00 MB/sec
/dev/sdag:
Timing buffered disk reads: 150 MB in 3.01 seconds = 49.82 MB/sec
/dev/sdh:
Timing buffered disk reads: 244 MB in 3.00 seconds = 81.27 MB/sec
/dev/sdah:
Timing buffered disk reads: 154 MB in 3.01 seconds = 51.23 MB/sec
/dev/sdi:
Timing buffered disk reads: 206 MB in 3.02 seconds = 68.32 MB/sec
/dev/sdai:
Timing buffered disk reads: 156 MB in 3.01 seconds = 51.90 MB/sec
/dev/sdj:
Timing buffered disk reads: 196 MB in 3.01 seconds = 65.09 MB/sec
/dev/sdaj:
Timing buffered disk reads: 158 MB in 3.02 seconds = 52.26 MB/sec
/dev/sdk:
Timing buffered disk reads: 236 MB in 3.02 seconds = 78.15 MB/sec
/dev/sdak:
Timing buffered disk reads: 162 MB in 3.02 seconds = 53.67 MB/sec
/dev/sdl:
Timing buffered disk reads: 216 MB in 3.01 seconds = 71.68 MB/sec
/dev/sdal:
Timing buffered disk reads: 220 MB in 3.01 seconds = 73.14 MB/sec
/dev/sdm:
Timing buffered disk reads: 202 MB in 3.01 seconds = 67.20 MB/sec
/dev/sdam:
Timing buffered disk reads: 180 MB in 3.02 seconds = 59.60 MB/sec
/dev/sdn:
Timing buffered disk reads: 230 MB in 3.00 seconds = 76.67 MB/sec
/dev/sdan:
Timing buffered disk reads: 182 MB in 3.01 seconds = 60.44 MB/sec
/dev/sdo:
Timing buffered disk reads: 218 MB in 3.01 seconds = 72.39 MB/sec
/dev/sdao:
Timing buffered disk reads: 176 MB in 3.01 seconds = 58.45 MB/sec
/dev/sdp:
Timing buffered disk reads: 202 MB in 3.00 seconds = 67.25 MB/sec
/dev/sdap:
Timing buffered disk reads: 182 MB in 3.03 seconds = 60.13 MB/sec
/dev/sdq:
Timing buffered disk reads: 230 MB in 3.01 seconds = 76.43 MB/sec
/dev/sdaq:
Timing buffered disk reads: 178 MB in 3.01 seconds = 59.23 MB/sec
/dev/sdr:
Timing buffered disk reads: 200 MB in 3.03 seconds = 65.98 MB/sec
/dev/sdar:
Timing buffered disk reads: 182 MB in 3.01 seconds = 60.41 MB/sec
/dev/sds:
Timing buffered disk reads: 238 MB in 3.02 seconds = 78.81 MB/sec
/dev/sdas:
Timing buffered disk reads: 146 MB in 3.00 seconds = 48.63 MB/sec
/dev/sdt:
Timing buffered disk reads: 198 MB in 3.03 seconds = 65.40 MB/sec
/dev/sdat:
Timing buffered disk reads: 228 MB in 3.02 seconds = 75.44 MB/sec
/dev/sdu:
Timing buffered disk reads: 200 MB in 3.02 seconds = 66.20 MB/sec
/dev/sdau:
Timing buffered disk reads: 184 MB in 3.02 seconds = 60.96 MB/sec
/dev/sdv:
Timing buffered disk reads: 210 MB in 3.02 seconds = 69.59 MB/sec
/dev/sdw:
Timing buffered disk reads: 184 MB in 3.02 seconds = 60.90 MB/sec
/dev/sdx:
Timing buffered disk reads: 218 MB in 3.02 seconds = 72.15 MB/sec
/dev/sdy:
Timing buffered disk reads: 232 MB in 3.00 seconds = 77.22 MB/sec
/dev/sdz:
Timing buffered disk reads: 200 MB in 3.02 seconds = 66.24 MB/sec
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