Andy Smith wrote:
> ...
For example, are *writes* to a 2 device RAID-0 approaching twice as
fast as to a single device? If not, are they any faster at all?
Are reads from a 2 device RAID-1 twice as fast as from a single
device? If there are benefits, how quickly do they degrade to
nothing as disks are added?
Server development where I work uses a 3-way mirror for system bits but
this would be a costly solution for any sort of real storage and/or write
performance.
Here's the last chunk of a pair of 120GB WD SATA-I drives, SiI3112 chipset,
sata_sil driver, 2.4.32 kernel. A three or four way stripe should get
proportionally more provided separate controller channels and of course,
risk scales with performance.
[16:05] abit:~ > cat /proc/mdstat | head -7
Personalities : [raid0] [raid1] [raid5]
read_ahead 1024 sectors
md14 : active raid0 sdb13[1] sda13[0]
114575616 blocks 32k chunks
md13 : active raid1 sdb12[1] sda12[0]
20113216 blocks [2/2] [UU]
[16:06] abit:~ > hdparm -tT /dev/{md14,sd{a,b}13,md13,sd{a,b}12}
/dev/md14:
Timing buffer-cache reads: 1908 MB in 2.00 seconds = 954.00 MB/sec
Timing buffered disk reads: 272 MB in 3.01 seconds = 90.37 MB/sec
/dev/sda13:
Timing buffer-cache reads: 1904 MB in 2.00 seconds = 952.00 MB/sec
Timing buffered disk reads: 156 MB in 3.01 seconds = 51.83 MB/sec
/dev/sdb13:
Timing buffer-cache reads: 1912 MB in 2.00 seconds = 956.00 MB/sec
Timing buffered disk reads: 136 MB in 3.00 seconds = 45.33 MB/sec
/dev/md13:
Timing buffer-cache reads: 1876 MB in 2.00 seconds = 938.00 MB/sec
Timing buffered disk reads: 164 MB in 3.00 seconds = 54.67 MB/sec
/dev/sda12:
Timing buffer-cache reads: 1904 MB in 2.00 seconds = 952.00 MB/sec
Timing buffered disk reads: 166 MB in 3.02 seconds = 54.97 MB/sec
/dev/sdb12:
Timing buffer-cache reads: 1892 MB in 2.00 seconds = 946.00 MB/sec
Timing buffered disk reads: 146 MB in 3.00 seconds = 48.67 MB/sec
[16:08] abit:~ >
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