ext3 zerofree option and RedHat back port?

Ric Wheeler rwheeler at redhat.com
Wed Sep 24 11:19:02 UTC 2008


Ulf Zimmermann wrote:
>> -----Original Message-----
>> From: Eric Sandeen [mailto:sandeen at redhat.com]
>> Sent: 09/23/2008 20:30
>> To: Ulf Zimmermann
>> Cc: Theodore Tso; ext3-users at redhat.com
>> Subject: Re: ext3 zerofree option and RedHat back port?
>>
>> Ulf Zimmermann wrote:
>>
>>     
>>> Reason I asked is this. We use currently 3Par S400 and E200 as SAN
>>> arrays. The new T400 and T800 has a built in chip to do more
>>>       
>> intelligent
>>     
>>> thin provisioning but I believe even the S400 and E200 we have will
>>>       
>> free
>>     
>>> on the SAN level a block of a thin provisioned volume if it gets
>>>       
>> zero'ed
>>     
>>> out. Haven't gotten around yet to test it, but I am planning on. We
>>>       
>> are
>>     
>>> currently using 3 different file system types, one is a propriety
>>>       
>> from
>>     
>>> Onstor for their Bobcats (NFS/CIFS heads) where I believe I have
>>> observed just freeing of SAN level blocks. The two other are EXT3
>>>       
> and
>   
>>> OCFS2.
>>>       
>> Ok, so you really want to zero the unused blocks in-place, and e2image
>> writing out a new sparsified image isn't a ton of help.
>>
>> The tool does that, I guess - but only on an unmounted or RO-mounted
>> filesystem, right?  (plus I'd triple-check that it's doing things
>> correctly, opening a block device and splatting zeros around, one
>>     
> hopes
>   
>> that it is!)
>>
>> But in any case the util itself is simple enough that building (or
>>     
> even
>   
>> packaging) for fedora/EPEL should be trivial.
>>
>> (FWIW, there is work upstream for filesystems to actually communicate
>> freed blocks to the underlying storage, just for this purpose...)
>>
>> -Eric
>>     
>
> I am going to try it out by hand. Create a thin provisioned volume,
> write random crap to it, then zero the blocks. See if that shrinks the
> physical allocated space.
>
> Ulf.
>
>
>   

Note that there is work on getting file systems to use the new TRIM (for 
S-ATA drives) and its equivalent proposed standard in T10 SCSI for 
arrays which will give you this automatically. David Woodhouse was 
pushing patches for TRIM, we are still thinking about the SCSI versions...

ric




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