[dm-crypt] Toggle read-only status of mapping

[dm-crypt] Toggle read-only status of mapping

[Dennis Birkholz]

Hello together,

currently it is possible to create a read-only mapping, but to me it
seems not possible to switch a rw mapping to a ro mapping or vice versa.
I use Software-RAID5->Luks->Ext4, do switch everything to read-only I
have to unmount, remove crypt mapping, create ro crypt mapping, mount
ro, switch raid to ro but it would be very nice if I only can switch
everything to ro.

Greets,
Dennis

Re: [dm-crypt] Toggle read-only status of mapping

[Milan Broz]

On 10/23/2012 06:55 AM, Dennis Birkholz wrote:
> currently it is possible to create a read-only mapping, but to me it
> seems not possible to switch a rw mapping to a ro mapping or vice versa.
> I use Software-RAID5->Luks->Ext4, do switch everything to read-only I
> have to unmount, remove crypt mapping, create ro crypt mapping, mount
> ro, switch raid to ro but it would be very nice if I only can switch
> everything to ro.

Hm. What is the reason for this exercise?
Switching to read only of top layer is not enough?

Anyway, I think we can add this option to "resize" command perhaps.
(Resize reloads mapping, so it can switch read-only flag as well.)

But it can be dangerous in some situations...
(And you can do it already using dmsetup but that's quite complicated.)

Milan

Re: [dm-crypt] Toggle read-only status of mapping

[Dennis Birkholz]

Am 25.10.2012 12:15, schrieb Milan Broz:
> On 10/23/2012 06:55 AM, Dennis Birkholz wrote:
>> currently it is possible to create a read-only mapping, but to me it
>> seems not possible to switch a rw mapping to a ro mapping or vice versa.
>> I use Software-RAID5->Luks->Ext4, do switch everything to read-only I
>> have to unmount, remove crypt mapping, create ro crypt mapping, mount
>> ro, switch raid to ro but it would be very nice if I only can switch
>> everything to ro.
> 
> Hm. What is the reason for this exercise?
> Switching to read only of top layer is not enough?

The reason is relatively easy: if my raid is read-only, it can crash and
I can reassemble it without a problem, but I cannot make it read-only if
the crypt-mapping is read-write.

> Anyway, I think we can add this option to "resize" command perhaps.
> (Resize reloads mapping, so it can switch read-only flag as well.)

That would be nice.

> But it can be dangerous in some situations...
> (And you can do it already using dmsetup but that's quite complicated.)

I will have a look into it.

Greets,
Dennis

Re: [dm-crypt] Toggle read-only status of mapping

[Milan Broz]

On 10/25/2012 04:50 PM, Dennis Birkholz wrote:
> Am 25.10.2012 12:15, schrieb Milan Broz:

> The reason is relatively easy: if my raid is read-only, it can crash and
> I can reassemble it without a problem, but I cannot make it read-only if
> the crypt-mapping is read-write.

If you raid is mapped read-write, and there is a device failure, it either
continue to be read-write (in degraded mode, that's exactly what "redundant"
in RAID means) or the array is lost completely (then you must deactivate
mapped device anyway).

And raid allows resync while read-write so I really do not see real
use for this switch. 
(And even read-only fs mount sometime means filesystem journal reply,
specifically after fs crash.)

Anyway, it is easily to be done with dmsetup:

  NAME=<your crypt device name>

  # load identical but read-only table into inactive slot
  dmsetup reload $NAME --readonly --table "$(dmsetup table --showkeys $NAME)"
  # switch tables
  dmsetup resume $NAME

Milan

Re: [dm-crypt] Toggle read-only status of mapping

[Dennis Birkholz]

Am 25.10.2012 17:20, schrieb Milan Broz:
> On 10/25/2012 04:50 PM, Dennis Birkholz wrote:
>> Am 25.10.2012 12:15, schrieb Milan Broz:
> 
>> The reason is relatively easy: if my raid is read-only, it can crash and
>> I can reassemble it without a problem, but I cannot make it read-only if
>> the crypt-mapping is read-write.
> 
> If you raid is mapped read-write, and there is a device failure, it either
> continue to be read-write (in degraded mode, that's exactly what "redundant"
> in RAID means) or the array is lost completely (then you must deactivate
> mapped device anyway).
> And raid allows resync while read-write so I really do not see real
> use for this switch. 
> (And even read-only fs mount sometime means filesystem journal reply,
> specifically after fs crash.)

I know how this works, just to wrap it up:
I have a RAID5 with 5 devices that are in a e-sata box where the e-sata
connection seems to be quite fragile: sometimes the port-multiplier in
the box starts to make problems and than one device after another drops
away. If my raid is read-write, the devices are marked unclean if they
drop out, if the raid is read-only, the devices are not marked unclean.
The contents of the raid is quite static so I only change something
every few days. If I just can change everything to rw before putting
something on the raid and switch it back to ro afterwards would be
really nice (and yes, I know of raid bitmap and I am going to change the
fragile e-sata box anyway in the future). Assembling the raid again
after it crashed and I restarted the e-sata box is the real problem here...

> Anyway, it is easily to be done with dmsetup:
> 
>   NAME=<your crypt device name>
> 
>   # load identical but read-only table into inactive slot
>   dmsetup reload $NAME --readonly --table "$(dmsetup table --showkeys $NAME)"
>   # switch tables
>   dmsetup resume $NAME

Thanks for the info!

Greets,
Dennis