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XROOTD-L  March 2008

XROOTD-L March 2008

Subject:

Re: oss.cache is too limited, any advice

From:

Patrick McGuigan <[log in to unmask]>

Date:

03 Mar 2008 15:32:45 -0600Mon, 03 Mar 2008 15:32:45 -0600

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text/plain

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text/plain (72 lines)

Hi Andy,

Is there a way to to enforce a read-only rule for particular data 
servers?  If this is possible, I can ensure that newly written data 
avoids the systems that need to be "LVM'ed", while allowing the existing 
data to be read.  I am curious if I can replicate data under this 
scenario using xrdcp, or will I still have to take the read-only systems 
off-line to move the data?

I am concerned about the granularity of recovery actions in our systems 
under LVM, but I need to support larger pathnames now.  The pathnames 
are being driven by the physics users and the use of metadata in the 
path components.

Patrick

Andrew Hanushevsky wrote:
> Hi Patrick,
> 
> You are quite right that the design of the cache does impose limits on 
> file names. The oss was designed almost 10 years ago when there were no 
> integrated LVM's and few that worked really well and users kept path 
> names to less that 200 characters. Over the years, as LVM's became 
> common, the oss turned into the "poor man's LVM". In general, we don't 
> recommend using it if you have ready access to a suitable LVM. While, 
> yes, you do give up some features (like fine-grained recoverability and 
> application-directed partition selection) the other
> limitations may be even more annoying. We'll make sure that this
> restriction is prominently mentioned in the manual.
> 
> The path you've chosen is about the only one that will work (i.e., 
> copying off the data and creating a single filesystem using an LVM).
> 
> Now, we do have some ideas on how to remove the pathname length 
> restriction but wonder if it's really worth the trouble of doing it, 
> given that LVM's provide practically the same basic features. Any thoughts?
> 
> Andy
> 
> P.S. What's the driving force for very long path names at your site?
> 
> 
> On Sat, 1 Mar 2008, Patrick McGuigan wrote:
> 
>> Hi,
>>
>> Some of our data servers have two disks and I am using the oss.cache 
>> directive to use both disks to support a single namespace.  However, 
>> it looks like the users of our applications have already run into a 
>> problem.  All of the files are stored in one directory (for a single 
>> cache) and the filename is the full namespace path with "/" 
>> substituted with "%".  Our problem arises from the fact that full 
>> namespace path is now limited to the leafname length of the filesystem 
>> (255 characters) when writing to the cache directory.
>>
>> I see a couple of ways to mediate the problem; removing one disk or 
>> using and LVM to create one drive in the OS.  I am curious if there 
>> are other alternatives?
>>
>> If I have to move to one disk, I would like to migrate the data in the 
>> existing caches to other data servers while I rework the existing 
>> system. What is the best way to migrate this data?  I am planning on 
>> taking the "problem" data server off-line and use xrdcp to move the 
>> data to the other servers.
>>
>> Regards,
>>
>> Patrick
>>


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