Re: Sleepy drives and MD RAID 6

Arg!!  Am I hitting some kind of blocking at the Linux kernel?? No
matter what I do, I can't seem to get the drives to spin up in
parallel.  Any ideas?

A simple test case trying to get two drives to spin up at once.
root@nas:~# hdparm -C /dev/sdh /dev/sdg
/dev/sdh:
 drive state is:  standby

/dev/sdg:
 drive state is:  standby

#Two terminal windows dd'ing sdg and sdh at the same time.
root@nas:~/dm_drive_sleeper# time dd if=/dev/sdh of=/dev/null bs=4096
count=1 iflag=direct
1+0 records in
1+0 records out
4096 bytes (4.1 kB) copied, 14.371 s, 0.3 kB/s

real   0m28.139s ############# WHY?! ################
user   0m0.000s
sys   0m0.000s

#A single drive spin-up
root@nas:~/dm_drive_sleeper# time dd if=/dev/sdh of=/dev/null bs=4096
count=1 iflag=direct
1+0 records in
1+0 records out
4096 bytes (4.1 kB) copied, 14.4212 s, 0.3 kB/s

real   0m14.424s
user   0m0.000s
sys   0m0.000s

On Tue, Aug 12, 2014 at 8:23 AM, Adam Talbot [off-list ref] wrote:

Thank you all for the input.  At this point I think I am going to write a
simple daemon to do dm power management. I still think this would be a good
feature set to roll into the driver stack, or madam-tools.

As far as wear and tear on the disks. Yes, starting and stopping the drives
shortens their life span. I don't trust my disks, regardless of
starting/stopping, that is why I run RAID 6. Lets say I use my NAS with it's
7 disks for 2 hours a day, 7 days a week @ 10 watts per drive.  The current
price for power in my area is $0.11 per kilowatt-hour. That comes out to be
$5.62 per year to run my drives for 2 hours, daily.  But if I run my drives
24/7 it would cost me $67.45/year.  Basically it would cost me an extra
$61.83/year to run the drives 24/7.  The 2TB 5400RPM SATA drives I have been
picking up from local surplus, or auction websites are costing me $40~$50,
including shipping and tax.  In other words I could buy a new disk every
8~10 months to replace failures and it would be the same cost. Drives don't
fail that fast, even if I was start/stopping them 10 times daily. This is
also completely ignoring the fact that drive prices are failing.  Sorry to
disappoint, but I am going to spin down my array and save some money.




On Tue, Aug 12, 2014 at 2:46 AM, Wilson, Jonathan
[off-list ref] wrote:

quoted

On Tue, 2014-08-12 at 07:55 +0200, Can Jeuleers wrote:

quoted

On 08/12/2014 03:21 AM, Larkin Lowrey wrote:

quoted

Also, leaving spin-up to the controller is
also not so hot since some controllers spin-up the drives sequentially
rather than in parallel.

Sequential spin-up is a feature to some, because it avoids large power
spikes.

I vaguely recall older drives had a jumper to set a delayed spin up so
they stayed in a low power (possibly un-spun up) mode when power was
applied and only woke up when a command was received (I think any
command, not a specific "wake up" one).

Also as mentioned some controllers may also only wake drives one after
the other, likewise mdriad does not care about the underlying
hardware/driver stack, only that it eventually responds, and even then I
believe it will happily wait till the end of time if no response or
error is propagated up the stack; hence the time out in scsi_device
stack not in the mdraid.

quoted

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