Re: Problems with BTRFS formatted disk

[Wang Yugui <wangyugui@e16-tech.com>]

Hi,

> On 2022/6/20 04:06, David C. Partridge wrote:
> > Yes write caching was enabled - I suspect that the way it worked was that on power fail the super-caps held the data until power was restored.
> >
> > Sadly it wasn't restored for a few weeks by which time the super-caps had lost their charge.
> 
> A little off-topic here, since I'm not familiar with how those hardware
> RAID controllers work.
> 
> Yep, those cards should have (super) caps to handle power loss, but for
>   SCSI SYNC CACHE commands, they should "the device server ensure that
> the specified logical blocks have their most recent data values recorded
> in non-volatile cache and/or on the medium."
> 
> Considering in a power loss event, the juice in those caps is definitely
> not enough to power those HDDs, it should at least have some
> non-volatile cache, like NAND, as backups.
> 
> But from sites I can found, it only states the card has 1024MiB cache
> memory.
> 
> Even with caps to keep the memory alive, it's still far from
> "non-volatile cache" required by SCSI spec.
> 
> Or is this a common practice in hardware RAID controller world to use
> volatile cache and break the SCSI spec requirement?


"non-volatile cache"  require a battery(inside or separated) and
Backup Flash(inside or separated).

For Microsemi Adaptec,
the battery and Backup Flash are in separated 'Flash Backup Module
AFM-700'.

For broadcom MegaRAID 9480/9580,
Backup Flash is inside the card,
but the battery is in separated CacheVault CVPM05.

For Dell, H730/H740 have battery/backup flash inside,
but H330 have no battery inside and we can not add battery to H330.

Best Regards
Wang Yugui (wangyugui@e16-tech.com)
2022/06/20