Re: bad block management

[Ric Wheeler <ric@emc.com>]

Toby Thain wrote:
> 
> On 4-Apr-08, at 2:58 PM, Ric Wheeler wrote:
>>
>> Toby Thain wrote:
>>> On 3-Apr-08, at 8:14 PM, Zan Lynx wrote:
>>>> On Tue, 2008-04-01 at 15:51 -0400, Jeff Mahoney wrote:
>>>>
>>>>> Ric's right about disk drives, though. They'll remap the bad sectors
>>>>> automatically at the hardware level. When you start to see bad sectors
>>>>> at the file system level, it means that the sectors reserved for
>>>>> remapping have been exhausted and you should replace the disk.
>>>>
>>>> There are a couple of cases where you can see bad block errors on a 
>>>> good
>>>> drive.
>>>>
>>>> If a block is written with a bad CRC for some reason...the write head
>>>> got a freak blip or it lost power as it was writing, or the data went
>>>> corrupt while sitting on disk, then it will read as a bad block, but
>>>> rewriting would fix it.
>>>>
>>>> A RAID media verify or a badblocks -n run can usually fix these.
>>> Only if your RAID uses CRCs (most don't).
>>> ZFS is the real answer to undetected corruption.
>>> --Toby
>>
>> Zan is right - even on a local drive, a write can repair some sectors 
>> with bad protection bits. All disks have per sector data protection 
>> (reed solomon encoding, etc) and there are lots of those bits per sector.
> 
> 
> That does not protect against writing bad data, only some errors 
> internal to drive. There is a long way to travel between CPU and drive. 
> Cable, controller, RAM, etc, etc, etc. ZFS protects the entire data path.
> 
> --Toby

If you want to protect the entire data path, you are looking at 
something like DIF which protects even more of the data path than ZFS 
since it adds a check from application space to the IO stack ;-)

ZFS does not export its protection bits up the stack.

ric


> 
>>
>> There is work on adding DIF (data integrity f?) which is extra bytes 
>> that arrays or local drives can store for application level 
>> protection. Martin Petersen has some good slides about this on linux:
>>
>> http://oss.oracle.com/projects/data-integrity/documentation/
>>
>> ZFS, for example, or more specifically its lvm layer, could use DIF to 
>> add this kind of protection.
>>
>> The other way to go is to use an enterprise class array - they all 
>> have multiple layers of data integrity baked in to deal with and 
>> correct these kind of errors.
>>
>> ric
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> 
>