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XROOTD-L  June 2008

XROOTD-L June 2008

Subject:

Re: oss.cache is too limited, any advice

From:

"Andrew Hanushevsky" <[log in to unmask]>

Date:

3 Jun 2008 18:01:59 -0700Tue, 3 Jun 2008 18:01:59 -0700

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text/plain (135 lines)

Hi Patrick,

We have a new development release on the web page. This release should 
address all of the problems you raised. Specifically, look at the "xa" 
option in the 'oss.cache' directive.

Andy

----- Original Message ----- 
From: "Patrick McGuigan" <[log in to unmask]>
To: "Andrew Hanushevsky" <[log in to unmask]>
Cc: <[log in to unmask]>
Sent: Monday, March 03, 2008 4:01 PM
Subject: Re: oss.cache is too limited, any advice


> Hi Andy,
>
> I'll let you know how the r/o migration attempt works when I try it later 
> tonight.
>
> I think it would be useful to remove the pathname limit under caching, 
> although I am biased ;)  I expect that ATLAS Tier3 sites will take a long 
> look at Scalla because of the integration with root and the more flexible 
> the software is, the more likely it will fit the perceived need.  I am 
> about to help setup a Tier3 site where the existing hardware provides 
> three disks per data server and would be willing to test implementations 
> there.
>
> Patrick
>
> Andrew Hanushevsky wrote:
>> Hi Patrick,
>>
>> Yes, you can do what you want to do. The most generc way is to export the 
>> paths as r/o (read/only). See the xrootd.export directive. You can do 
>> what you want, though we have never tried it. That is, having a server 
>> exporting a read/only path that you essentially want to duplicate on a 
>> read/write server(s). It probably will work. If not, there are a couple 
>> of other things you can do to force it to work. BTW to keep a single 
>> config file, simply bracket the exceptional export in an if/fi construct 
>> rereferencing that host. For instance,
>>
>> if <problem host>
>> xrootd.export /thepath r/o
>> else
>> xrootd.export /thepath
>> fi
>>
>> I understand your concern about granularity. That problem seems to plague 
>> many LVMs. The newest ones (i.e., zfs) try to address that issue. So, 
>> does that mean you think it's worth spending time removing the 255 path 
>> limit?
>>
>> Andy
>>
>>
>> On Mon, 3 Mar 2008, Patrick McGuigan wrote:
>>
>>> 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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