Re: Raid1 fast resync

[Philip Cameron <pecameron@attbi.com>]

Hi Alvin,

Thanks for the comments. The hardware includes two hot swappable  IO 
assemblies each of which includes a PCI bus, SCSI HBA and 3 hot swapable 
SCSI disk slots. All of the issues related to hot-swap have been 
resolved so I don't need to deal with SCSI and PCI bus issues (thankfully).

Because of the redundant IO assemblies, the system is not considered 
fully operational until either assembly can fail or be pulled at random. 
This means the resync must be complete. So the real deal is reducing the 
resync so that the system can be considered fully operational sooner. I 
am looking to reduce the multiple hour resync times to a few minutes.

Thanks again,
Phil Cameron
---------------------------

Alvin Oga wrote:

>hi ya philip
>
>On Wed, 1 Jan 2003, Philip Cameron wrote:
>
>  
>
>>Do any of you know of work that is going on to improve the raid 1 resync 
>>speed? I am starting to evaluate approaches to reducing the time to 
>>resync the disks.
>>
>>I am involved in a project on a fault tolerant x86 based server that has 
>>hot plugable scsi disks and PCI busses. There are two PCI busses and 3 
>>mirrrored sets of two disks. When a PCI bus is pulled all of the mirrors 
>>are broken so when the PCI bus is again inserted, all of the disks 
>>resync which takes hours. Also, during development the resync after 
>>crash really slows down debug.
>>    
>>
>
>without knowing more details ... here's some comments
>
>- when a pci card is pulled out, you stand a good chance that the cpu bios
>  will need to be reset ( re-saved )
>	- the machine goes into bios mode, whether you like it or not
>
>- since you are using scsi disks..
>	- when you pull out a scsi disk, the next time you reboot,
>	the scsi drives will be in different order if you lost sda
>	or lost sdb and you had sdc  which is now your new sdb
>
>- if you have 2 pci buses...
>	- put sda  on pci #1
>	and put sdb ( mirrored ) onto pci #2
>
>	- if any pci buss dies, your system can still function properly
>	since its mirror is on the other pci buss which is working
>
>- to avoid resyncing the raid data...
>	- use a 3rd (spare) disk local to the 2 disks
>	when one disk dies, the spare supposed to kick-in 
>
>	- have a spare system, that mirror's the "main" data,
>	and rebuild a spare disk on a constant basis so that
>	the "spare" can be plugged into the "main" data asap
>	whenver a disk dies that didnt have a spare previously
>
>	- when resyncing... you only need to "resync changed data"
>	( changed/new files for today ) and NOT the whole array
>	which might have a year's worth of data..
>
>	- yesterday's hot swap ( spare ) is ready to go anytime
>
>- if you want more raid interactivity and keyboard/ssh control,
>  renice the apps and raid parameters
>
>have fun
>alvin
>
>-
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