Re: [ovirt-users] effectiveness of "discard=unmap"

[Matthias Leopold]

Am 2018-02-12 um 13:40 schrieb Idan Shaby:
On Mon, Feb 12, 2018 at 1:55 PM, Matthias Leopold 
> wrote:


Hi Idan,

thanks for your answer. But i'm still confused, because i thought
that the content of /sys/block/dm-X/queue/discard* in the VM OS
should depend on the setting of the "discard=(unmap|ignore)" setting
in the qemu-kvm command. Unexpectedly it's the same in both cases
(it's >0, saying discard is 'on'). I was then trying to inquire
about the TRIM/UNMAP capability of block devices in the VM with
"sdparm -p lbp /dev/sdx", but i always get "Logical block
provisioning (SBC) mode subpage failed".

The file /sys/block/dm-X/queue/discard_max_bytes in sysfs tells you 
whether your underlying storage supports discard.
The flag discard=unmap of the VM in qemu means that qemu will not throw 
away the UNMAP commands comming from the guest OS (by default it does 
throw them away).

 From what I know, the file in sysfs and the VM flag are not related.


Thank you, i will now finally accept it ;-)

Just for the records: This is where i got my info from: 
https://chrisirwin.ca/posts/discard-with-kvm/


Matthias
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Re: [ovirt-users] effectiveness of "discard=unmap"

[Idan Shaby]

On Mon, Feb 12, 2018 at 1:55 PM, Matthias Leopold <
matthias.leop...@meduniwien.ac.at> wrote:

> Hi Idan,
>
> thanks for your answer. But i'm still confused, because i thought that the
> content of /sys/block/dm-X/queue/discard* in the VM OS should depend on the
> setting of the "discard=(unmap|ignore)" setting in the qemu-kvm command.
> Unexpectedly it's the same in both cases (it's >0, saying discard is 'on').
> I was then trying to inquire about the TRIM/UNMAP capability of block
> devices in the VM with "sdparm -p lbp /dev/sdx", but i always get "Logical
> block provisioning (SBC) mode subpage failed".
>
The file /sys/block/dm-X/queue/discard_max_bytes in sysfs tells you whether
your underlying storage supports discard.
The flag discard=unmap of the VM in qemu means that qemu will not throw
away the UNMAP commands comming from the guest OS (by default it does throw
them away).
>From what I know, the file in sysfs and the VM flag are not related.

>
> I know i can (and should) look at the storage array to see if TRIM/UNMAP
> _actually_ works, but documentation (https://www.kernel.org/doc/Do
> cumentation/block/queue-sysfs.txt) says it should be visible beforehand
> to my understanding.
>
> Regards
> Matthias
>
> Am 2018-02-11 um 08:55 schrieb Idan Shaby:
>
>> Hi Matthias,
>>
>> When the guest executes a discard call of any variation (fstrim,
>> blkdiscard, etc.), the underlying thinly provisioned LUN is the one that
>> changes -  it returns the unused blocks to the storage array and gets
>> smaller.
>> Therefore, no change is visible to the guest OS.
>> If you want to check what has changed, go to the storage array and check
>> what's the size of the underlying thinly provisioned LUN before and after
>> the discard call.
>>
>> The answer for your question and some more information can be found in
>> the feature page [1] (needs a bit of an update, but most of it is still
>> relevant).
>> If you got any further questions, please don't hesitate to ask.
>>
>>
>> Regards,
>> Idan
>>
>> [1] Pass discard from guest to underlying storage -
>> https://www.ovirt.org/develop/release-management/features/st
>> orage/pass-discard-from-guest-to-underlying-storage/
>>
>> On Thu, Feb 8, 2018 at 2:08 PM, Matthias Leopold <
>> matthias.leop...@meduniwien.ac.at > iwien.ac.at>> wrote:
>>
>> Hi,
>>
>> i'm sorry to bother you again with my ignorance of the DISCARD
>> feature for block devices in general.
>>
>> after finding several ways to enable "discard=unmap" for oVirt disks
>> (via standard GUI option for iSCSI disks or via "diskunmap" custom
>> property for Cinder disks) i wanted to check in the guest for the
>> effectiveness of this feature. to my surprise i couldn't find a
>> difference between Linux guests with and without "discard=unmap"
>> enabled in the VM. "lsblk -D" reports the same in both cases and
>> also fstrim/blkdiscard commands appear to work with no difference.
>> Why is this? Do i have to look at the underlying storage to find out
>> what really happens? Shouldn't this be visible in the guest OS?
>>
>> thx
>> matthias
>>
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>> 
>>
>>
>>
> --
> Matthias Leopold
> IT Systems & Communications
> Medizinische Universität Wien
> Spitalgasse 23 / BT 88 /Ebene 00
> A-1090 Wien
> Tel: +43 1 40160-21241
> Fax: +43 1 40160-921200
>
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Re: [ovirt-users] effectiveness of "discard=unmap"

[Idan Shaby]

Hi Matthias,

When the guest executes a discard call of any variation (fstrim,
blkdiscard, etc.), the underlying thinly provisioned LUN is the one that
changes -  it returns the unused blocks to the storage array and gets
smaller.
Therefore, no change is visible to the guest OS.
If you want to check what has changed, go to the storage array and check
what's the size of the underlying thinly provisioned LUN before and after
the discard call.

The answer for your question and some more information can be found in the
feature page [1] (needs a bit of an update, but most of it is still
relevant).
If you got any further questions, please don't hesitate to ask.


Regards,
Idan

[1] Pass discard from guest to underlying storage -
https://www.ovirt.org/develop/release-management/features/storage/pass-discard-from-guest-to-underlying-storage/

On Thu, Feb 8, 2018 at 2:08 PM, Matthias Leopold <
matthias.leop...@meduniwien.ac.at> wrote:

> Hi,
>
> i'm sorry to bother you again with my ignorance of the DISCARD feature for
> block devices in general.
>
> after finding several ways to enable "discard=unmap" for oVirt disks (via
> standard GUI option for iSCSI disks or via "diskunmap" custom property for
> Cinder disks) i wanted to check in the guest for the effectiveness of this
> feature. to my surprise i couldn't find a difference between Linux guests
> with and without "discard=unmap" enabled in the VM. "lsblk -D" reports the
> same in both cases and also fstrim/blkdiscard commands appear to work with
> no difference. Why is this? Do i have to look at the underlying storage to
> find out what really happens? Shouldn't this be visible in the guest OS?
>
> thx
> matthias
>
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[ovirt-users] effectiveness of "discard=unmap"

[Matthias Leopold]

Hi,

i'm sorry to bother you again with my ignorance of the DISCARD feature 
for block devices in general.


after finding several ways to enable "discard=unmap" for oVirt disks 
(via standard GUI option for iSCSI disks or via "diskunmap" custom 
property for Cinder disks) i wanted to check in the guest for the 
effectiveness of this feature. to my surprise i couldn't find a 
difference between Linux guests with and without "discard=unmap" enabled 
in the VM. "lsblk -D" reports the same in both cases and also 
fstrim/blkdiscard commands appear to work with no difference. Why is 
this? Do i have to look at the underlying storage to find out what 
really happens? Shouldn't this be visible in the guest OS?


thx
matthias

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