Character limit for rbd block device

Character limit for rbd block device

[brady]

Hello,
     I have a general question. Is there a specific character limit for 
rbd block devices? In attempting to map a block device with a name that 
is 36 characters or more, I am getting the following error:

rbd map (36+ character name) --user admin --secret /etc/ceph/keyring.key
add failed: (2) No such file or directory

     When using the same command to map a device with 35 characters or 
less, the device is mapped as expected. Any insight into this issue 
would be greatly appreciated.

-Brady

Re: Character limit for rbd block device

[Josh Durgin]

On 11/26/2012 04:10 PM, brady wrote:
> Hello,
>      I have a general question. Is there a specific character limit for
> rbd block devices? In attempting to map a block device with a name that
> is 36 characters or more, I am getting the following error:
>
> rbd map (36+ character name) --user admin --secret /etc/ceph/keyring.key
> add failed: (2) No such file or directory
>
>      When using the same command to map a device with 35 characters or
> less, the device is mapped as expected. Any insight into this issue
> would be greatly appreciated.
>
> -Brady

Which kernel version are you using?

A year ago there was a 40-char limit on objects names in the kernel's
libceph module, hence the failure at 36 char rbd image names (36 char
name + ".rbd" is the format 1 header object name).

This was extended to a 96-char limit in linux 3.2. There's no real limit
in userspace still (http://tracker.newdream.net/issues/1701).

Josh

Re: Character limit for rbd block device

[brady]

On 11/26/2012 04:52 PM, Josh Durgin wrote:
> On 11/26/2012 04:10 PM, brady wrote:
>> Hello,
>>      I have a general question. Is there a specific character limit for
>> rbd block devices? In attempting to map a block device with a name that
>> is 36 characters or more, I am getting the following error:
>>
>> rbd map (36+ character name) --user admin --secret /etc/ceph/keyring.key
>> add failed: (2) No such file or directory
>>
>>      When using the same command to map a device with 35 characters or
>> less, the device is mapped as expected. Any insight into this issue
>> would be greatly appreciated.
>>
>> -Brady
>
> Which kernel version are you using?
>
> A year ago there was a 40-char limit on objects names in the kernel's
> libceph module, hence the failure at 36 char rbd image names (36 char
> name + ".rbd" is the format 1 header object name).
>
> This was extended to a 96-char limit in linux 3.2. There's no real limit
> in userspace still (http://tracker.newdream.net/issues/1701).
>
> Josh
>
Thanks, Josh. This information was exactly what we were looking for. 
Looks like a version issue that we will need to work around. Thanks again!

Brady
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Re: Character limit for rbd block device

[Alex Elder]

On 11/26/2012 06:10 PM, brady wrote:
> Hello,
>     I have a general question. Is there a specific character limit for
> rbd block devices? In attempting to map a block device with a name that
> is 36 characters or more, I am getting the following error:
> 
> rbd map (36+ character name) --user admin --secret /etc/ceph/keyring.key
> add failed: (2) No such file or directory

We can set the maximum supported length almost arbitrarily high,
although we use the image name in a path component.  Linux limits
such file names to 255 bytes normally.

As Josh said, the limit is higher than what you mention.  Do
you have any thoughts on what constitutes a reasonable limit?
Are you using some sort of naming convention that produces
your 36+ byte image names?

Thanks.

					-Alex

>     When using the same command to map a device with 35 characters or
> less, the device is mapped as expected. Any insight into this issue
> would be greatly appreciated.
> 
> -Brady
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