[PATCH 0 of 13] LVM add 'mirrored' log type

[PATCH 0 of 13] LVM add 'mirrored' log type

[Jonathan Brassow]

I've fixed the segfaults Taka reported.  I've also begun to go through
and fix the remaining issues with handling device failures in the
various different scenarios.  I could use some help testing all the
different variations.  I've described below some of the different
permutations that will now exist.  I will be going through and testing
these as well.

Device failure handling policies:
=================================
The following policies are available for both mirror logs and mirror
images...
"remove" (rm):
Take out the failed device from the LV and do nothing else.  IOW, just
make things consistent again.

"replace" (rp):
Remove the failed device and attempt to replace it with available space
in the VG.  IOW, try to recreate what was lost.

Device failure scenarios:
=========================
"2devs":
You will have to use '--alloc anywhere' to get the mirrored log to be
placed on the same devices as the mirror images.  Failing a single
device will result in a failure of an image in the mirrored log and the
mirror itself - simultaneously.  Obviously, a "replace" policy will
fail, but the results should be the same as "remove".  The final result
should be a linear device.

"3devs":
'--alloc anywhere' is still necessary.  Failing a single leg can affect
the mirror, its mirrored log, or both.  The "remove" policy can leave a
lone linear LV or a mirror with a linear log.  The "replace" policy
should be able to restore the LV.  It is also possible to test 3-way
mirrors and fail one of the legs - presumably leaving a 2-way mirror and
a mirrored log.

"5devs":
Gives the ability to have the mirrored log on separate devices from the
mirror images and be able to "replace" the device.

So, the it is necessary to test all of the fault scenarios in each cell
of the following matrix.  Any help would be most welcome.

          | 2devs | 3devs | 5devs |
----------|-------|-------|-------|
rm_l,rm_i |       |       |       |
rm_l,rp_i |       |       |       |
rp_l,rm_i |       |       |       |
rp_l,rp_i |       |       |       |
-----------------------------------

 brassow

[PATCH 0 of 13] LVM add 'mirrored' log type

[Jonathan Brassow]

I've fixed the segfaults Taka reported.  I've also begun to go through
and fix the remaining issues with handling device failures in the
various different scenarios.  I could use some help testing all the
different variations.  I've described below some of the different
permutations that will now exist.  I will be going through and testing
these as well.

Device failure handling policies:
=================================
The following policies are available for both mirror logs and mirror
images...
"remove" (rm):
Take out the failed device from the LV and do nothing else.  IOW, just
make things consistent again.

"replace" (rp):
Remove the failed device and attempt to replace it with available space
in the VG.  IOW, try to recreate what was lost.

Device failure scenarios:
=========================
"2devs":
You will have to use '--alloc anywhere' to get the mirrored log to be
placed on the same devices as the mirror images.  Failing a single
device will result in a failure of an image in the mirrored log and the
mirror itself - simultaneously.  Obviously, a "replace" policy will
fail, but the results should be the same as "remove".  The final result
should be a linear device.

"3devs":
'--alloc anywhere' is still necessary.  Failing a single leg can affect
the mirror, its mirrored log, or both.  The "remove" policy can leave a
lone linear LV or a mirror with a linear log.  The "replace" policy
should be able to restore the LV.  It is also possible to test 3-way
mirrors and fail one of the legs - presumably leaving a 2-way mirror and
a mirrored log.

"5devs":
Gives the ability to have the mirrored log on separate devices from the
mirror images and be able to "replace" the device.

So, the it is necessary to test all of the fault scenarios in each cell
of the following matrix.  Any help would be most welcome.

          | 2devs | 3devs | 5devs |
----------|-------|-------|-------|
rm_l,rm_i |       |       |       |
rm_l,rp_i |       |       |       |
rp_l,rm_i |       |       |       |
rp_l,rp_i |       |       |       |
-----------------------------------

 brassow

[PATCH 0 of 13] LVM add 'mirrored' log type

[Jonathan Brassow]

I'm also testing the other approach; that is, allowing lvconvert to  
take non-top-level LVs (e.g. "lv_mlog").  It seems to work just fine.   
We might want to strongly consider allowing this.

There are drawbacks to this approach too, however.  For example, when  
one of the devices of the log fails, it will be treated as an 'image'  
failure, and not a 'log' failure (I can fix that though by checking  
its lv->status).  If we decide to go this route, we will have to  
decide if this is the behavior we want or not.  (Once the log is  
linear, it will behave as you would expect - either being replaced or  
removed according to the log policy.)

  brassow

On Feb 17, 2010, at 11:55 AM, Jonathan Brassow wrote:

> I've fixed the segfaults Taka reported.  I've also begun to go through
> and fix the remaining issues with handling device failures in the
> various different scenarios.  I could use some help testing all the
> different variations.  I've described below some of the different
> permutations that will now exist.  I will be going through and testing
> these as well.
>
> Device failure handling policies:
> =================================
> The following policies are available for both mirror logs and mirror
> images...
> "remove" (rm):
> Take out the failed device from the LV and do nothing else.  IOW, just
> make things consistent again.
>
> "replace" (rp):
> Remove the failed device and attempt to replace it with available  
> space
> in the VG.  IOW, try to recreate what was lost.
>
> Device failure scenarios:
> =========================
> "2devs":
> You will have to use '--alloc anywhere' to get the mirrored log to be
> placed on the same devices as the mirror images.  Failing a single
> device will result in a failure of an image in the mirrored log and  
> the
> mirror itself - simultaneously.  Obviously, a "replace" policy will
> fail, but the results should be the same as "remove".  The final  
> result
> should be a linear device.
>
> "3devs":
> '--alloc anywhere' is still necessary.  Failing a single leg can  
> affect
> the mirror, its mirrored log, or both.  The "remove" policy can  
> leave a
> lone linear LV or a mirror with a linear log.  The "replace" policy
> should be able to restore the LV.  It is also possible to test 3-way
> mirrors and fail one of the legs - presumably leaving a 2-way mirror  
> and
> a mirrored log.
>
> "5devs":
> Gives the ability to have the mirrored log on separate devices from  
> the
> mirror images and be able to "replace" the device.
>
> So, the it is necessary to test all of the fault scenarios in each  
> cell
> of the following matrix.  Any help would be most welcome.
>
>          | 2devs | 3devs | 5devs |
> ----------|-------|-------|-------|
> rm_l,rm_i |       |       |       |
> rm_l,rp_i |       |       |       |
> rp_l,rm_i |       |       |       |
> rp_l,rp_i |       |       |       |
> -----------------------------------
>
> brassow
>
> --
> lvm-devel mailing list
> lvm-devel at redhat.com
> https://www.redhat.com/mailman/listinfo/lvm-devel

[PATCH 0 of 13] LVM add 'mirrored' log type

[Takahiro Yasui]

Jonathan Brassow wrote:
> I've fixed the segfaults Taka reported.  I've also begun to go through
> and fix the remaining issues with handling device failures in the
> various different scenarios.  I could use some help testing all the
> different variations.  I've described below some of the different
> permutations that will now exist.  I will be going through and testing
> these as well.

'lvconvert --repair' doen't work and canuses *deadlock*. when a VG contains
two PVs and one of PVs failed; This is a situation that a mirror leg and
one of mirrored logs broke at the same time.

When one of mirrored logs broke, I/Os to mirrored log are blocked and
kmirrord can't make forward until the log is fixed. However, lvocnvert
command tries to fix mirror legs before a mirrored log if a mirror leg
also broke.

During the procedure to fix a mirror leg, lv_suspend() is called in
_remove_mirror_images() and lv_suspend() which calls kernel function,
dm_suspend(), doesn't finish. Rough call trace is as follows:

  dm_suspend()
    mirror_presuspend()
      flush_workqueue()
        -> waiting kmirrored
           -> waiting I/O to its log

I think that an error recovery of a log device should be done before
mirror legs are recovered.

Thanks,
Taka

[PATCH 0 of 13] LVM add 'mirrored' log type

[Takahiro Yasui]

Jonathan Brassow wrote:
> I'm also testing the other approach; that is, allowing lvconvert to take
> non-top-level LVs (e.g. "lv_mlog").  It seems to work just fine.  We
> might want to strongly consider allowing this.
> 
> There are drawbacks to this approach too, however.  For example, when
> one of the devices of the log fails, it will be treated as an 'image'
> failure, and not a 'log' failure (I can fix that though by checking its
> lv->status).  If we decide to go this route, we will have to decide if
> this is the behavior we want or not.  (Once the log is linear, it will
> behave as you would expect - either being replaced or removed according
> to the log policy.)

This approach requires two threads in dmeventd; it is for a top-level LV,
and another is for a mirrored log. If a leg and mirrored log failed at
the same time, a mirrored log needs to be recovered before a top-level LV.
I think this approach is also good if we can handle this situation.

Thanks,
Taka