Spinning down idle disks?

Spinning down idle disks?

[Roy Sigurd Karlsbakk]

Hi all

Is it possible somehow to have linux spin down idle disks in an MD raid as to use MD for a MAID (massive array of idle disks)? I tried to monitor an idle raid with blktrace, and it seems the array (and its members) is accessed every two seconds for some reason. The array used with for the testing is a idle, degraded raid-5.

roy

root@Mathsterk:~# blktrace -d /dev/md0 -o - | blkparse -i -
  9,0    2        1     0.000000000 26793  A   W 5368711168 + 8 <- (252,2) 0
  9,0    2        2     0.000000879 26793  Q   W 5368711168 + 8 [(null)]
  9,0    1        3     0.020063084   372  C   W 5368711168 [0]
  9,0    2        3     0.020113389 26793  A WBS 0 + 0 <- (252,2) 0
  9,0    2        4     0.020114118 26793  Q WBS [kworker/u:1]
  9,0    0        1     1.999791074  6948  A WBS 0 + 0 <- (252,2) 0
  9,0    0        2     1.999792301  6948  Q WBS [kworker/u:2]
  9,0    0        3     2.000000284  6948  A   W 5368711168 + 8 <- (252,2) 0
  9,0    0        4     2.000001050  6948  Q   W 5368711168 + 8 [kworker/u:2]
  9,0    0        5     2.020065947  6948  A WBS 0 + 0 <- (252,2) 0
  9,0    0        6     2.020066632  6948  Q WBS [kworker/u:2]
  9,0    1        4     2.020018632   372  C   W 5368711168 [0]
  9,0    0        7     3.999790266 26793  A WBS 0 + 0 <- (252,2) 0
  9,0    0        8     3.999791552 26793  Q WBS [kworker/u:1]
  9,0    0        9     4.000000435 26793  A   W 5368711168 + 8 <- (252,2) 0
  9,0    0       10     4.000001135 26793  Q   W 5368711168 + 8 [kworker/u:1]
  9,0    0       11     4.020027371 26793  A WBS 0 + 0 <- (252,2) 0
  9,0    0       12     4.020028051 26793  Q WBS [kworker/u:1]
  9,0    1        5     4.019973885   372  C   W 5368711168 [0]
  9,0    0       13     5.999788009  6948  A WBS 0 + 0 <- (252,2) 0
  9,0    0       14     5.999789265  6948  Q WBS [kworker/u:2]
  9,0    0       15     5.999999943  6948  A   W 5368711168 + 8 <- (252,2) 0
  9,0    0       16     6.000000676  6948  Q   W 5368711168 + 8 [kworker/u:2]
  9,0    0       17     6.019936327  6948  A WBS 0 + 0 <- (252,2) 0
  9,0    0       18     6.019937059  6948  Q WBS [kworker/u:2]
  9,0    1        6     6.019883619   372  C   W 5368711168 [0]
  9,0    2        5     8.019890546   372  C   W 5368711168 [0]
  9,0    0       19     7.999785408 26793  A WBS 0 + 0 <- (252,2) 0
  9,0    0       20     7.999786808 26793  Q WBS [kworker/u:1]
  9,0    0       21     7.999997589 26793  A   W 5368711168 + 8 <- (252,2) 0
  9,0    0       22     7.999998282 26793  Q   W 5368711168 + 8 [kworker/u:1]
  9,0    0       23     8.019948314 26793  A WBS 0 + 0 <- (252,2) 0
  9,0    0       24     8.019949033 26793  Q WBS [kworker/u:1]


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Vennlige hilsener / Best regards

roy
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Roy Sigurd Karlsbakk
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roy@karlsbakk.net
http://blogg.karlsbakk.net/
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Re: Spinning down idle disks?

[Vincent Pelletier]

Le dimanche 26 mai 2013 12:49:54, Roy Sigurd Karlsbakk a écrit :
> Is it possible somehow to have linux spin down idle disks in an MD raid as
> to use MD for a MAID (massive array of idle disks)? I tried to monitor an
> idle raid with blktrace, and it seems the array (and its members) is
> accessed every two seconds for some reason. The array used with for the
> testing is a idle, degraded raid-5.

FWIW, there is a small daemon to spin disks down in a - to me - clever way: 
only reads from device reset spindown timeout. Writes are kept in cache until 
an explicit flush happens.

http://noflushd.sourceforge.net/

This allows skipping superblock refreshes (FS- and MD-level).
It has a large drawback when using it for MD (and other composite devices): it 
doesn't look for slave devices, so if you tell it to control sda and sdb and 
you have md0 on top of both, it will eventually spin down sda and sdb, only to 
manually flush md0, spinning both up again.

I've implemented a quick-hack workaround for this:

https://github.com/vpelletier/pynoflushd

Both implementation have the drawback of increasing the frequency of writes to 
the actual disk: as the daemon take over dirty_writeback_centisecs's job using 
userspace-available flush methods (mine using BLKFLSBUF ioctl, the original 
using fsync on block device), something gets flushed which is usually not (I 
wandered a bit in kernel code without finding how writeback code handles this 
timeout).

I think it would be nice to have an equivalent of dirty_writeback_centisecs at 
device granularity, so that one doesn't have to delegate flushing to a 
userspace daemon.

Regards,
-- 
Vincent Pelletier
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