[dm-crypt] Elevator / dm-crypt

[dm-crypt] Elevator / dm-crypt

[Heinz Diehl]

Hi,

assuming /dev/sda1 is an LUKS/dmcrypt encrypted partition. The default
elevator is "noop" (given in kernel command-line), but "cfq" is set
for /dev/sda in /sys/block/sda/queue/scheduler.

What elevator is used for the underlying /dev/mapper/dm-0 (sda1)?

sys/block/dm-0/queue/scheduler contains "none", btw.

Thanks,
Heinz.

Re: [dm-crypt] Elevator / dm-crypt

[Milan Broz]

On 09/02/2012 06:35 PM, Heinz Diehl wrote:
> assuming /dev/sda1 is an LUKS/dmcrypt encrypted partition. The default
> elevator is "noop" (given in kernel command-line), but "cfq" is set
> for /dev/sda in /sys/block/sda/queue/scheduler.
> 
> What elevator is used for the underlying /dev/mapper/dm-0 (sda1)?

Partitions just use whole device elevator.
And it should be default from your kernel parameter configuration
if not set by some script later... is that param correct?
Anyway, you can just switch it later using
echo "noop">/sys/block/sda/queue/scheduler

Device-mapper devices (except request based - which is currently multipath only)
do not use own elevator at all. So only elevator for underlying device is important.

BTW you can easily check it with lsblk -t.

Or explicitly name fields - here is example of my nb with SSD
(default sched. is noop for me):

# lsblk -o NAME,TYPE,SCHED /dev/sdb
NAME                         TYPE  SCHED
sdb                          disk  noop
├─sdb1                       part  noop
├─sdb2                       part  noop
│ └─sdb2_crypt (dm-0)        crypt 
│   ├─mnbook-data (dm-1)     lvm   
│   ├─nbook-swap (dm-2)      lvm   
│   └─nbook-root64 (dm-3)    lvm   
└─sdb3                       part  noop

> sys/block/dm-0/queue/scheduler contains "none", btw.

Yes, it is irrelevant for dm-crypt, there is no elevator.
(If you ask why there is <dev>/queue dir then, the because someone
decided to put some important attributes there which applies even
when elevator is not set. No idea why...)

Milan

Re: [dm-crypt] Elevator / dm-crypt

[Heinz Diehl]

On 02.09.2012, Milan Broz wrote: 

[....]

Thanks a lot, Milan! Now it's all clear to me.

My main intention is to set "noop" as the default elevator, and switch
to cfq for the harddisk via rc.local when booting (it's a conventional
rotating drive, no SSD). This has the effect that all USB drives
plugged in later will get "noop" as their active scheduler, since it's
the default, while all partitions on the harddisk have "cfq".

This way, I avoid hair-pulling work with udev/systemd to have "noop"
for all non-rotational drives plugged in :-)

Re: [dm-crypt] Elevator / dm-crypt

[Milan Broz]

On 09/02/2012 07:45 PM, Heinz Diehl wrote:
> On 02.09.2012, Milan Broz wrote: 
> 
> [....]
> 
> Thanks a lot, Milan! Now it's all clear to me.
> 
> My main intention is to set "noop" as the default elevator, and switch
> to cfq for the harddisk via rc.local when booting (it's a conventional
> rotating drive, no SSD). This has the effect that all USB drives
> plugged in later will get "noop" as their active scheduler, since it's
> the default, while all partitions on the harddisk have "cfq".
> 
> This way, I avoid hair-pulling work with udev/systemd to have "noop"
> for all non-rotational drives plugged in :-)

You can also use udev rules to do this
(not tested but something like):

Add /etc/udev/rules.d/20-elevator.rules 

# set cfq for all rotational devices
SUBSYSTEM=="block", ACTION=="add", ATTR{queue/rotational}=="1", ATTR{queue/scheduler}="cfq"

(or perhaps reverse is better:)
SUBSYSTEM=="block", ACTION=="add", ATTR{queue/rotational}=="0", ATTR{queue/scheduler}="noop"

.. or/and for USB
# set noop for usb disks
SUBSYSTEMS=="usb", ACTION=="add", KERNEL=="sd*[!0-9]|sr*", ATTR{queue/scheduler}="noop"

(Btw I think "deadline" could be better for USB.)

Milan