[rescue] [FWIW] RaSCSI

[Peter Corlett]

On Mon, Feb 21, 2022 at 03:02:53PM -0600, Lionel Peterson wrote:
> Let's not forget the absolute performance ceiling imposed by using a
> MicroSD card for the actual storage medium.

There are good reasons to be wary of (micro)SD cards, but performance is not
one of them: any non-counterfeit card from a reputable vendor will readily
saturate a SCSI-II bus. On a random Pi4 I've got lying around here:

abuse at babbage:~$ sudo hdparm -t /dev/mmcblk0

/dev/mmcblk0:
 HDIO_DRIVE_CMD(identify) failed: Invalid argument
  Timing buffered disk reads: 128 MB in  3.02 seconds =  42.32 MB/sec

The reason to be wary is the write endurance. However, a Raspberry Pi also
has USB and Ethernet, so one could USB-attach a SATA SSD, or use a NAS. The
Pi4 can saturate its Gigabit Ethernet port: is ~115MB/s fast enough for you?

> It would be interesting to see performance comparisons between spinning
> SCA SCSI drives and any of these MicroSD card-based alternatives.

Modern flash-based solutions will wipe the floor with those 20-year old SCSI
spinners for most purposes because the disks have sub-millisecond latency
and there's a much larger RAM cache.

> An SCA sled that takes SATA SSD would be great, anyone know about any?

Unless you're determined to reuse an old SCA backplane, you'd be better off
finding a supported SATA or SAS controller and plug the SATA SSDs into that.
Cards based on the LSI SAS2008 chipset are plentiful and cheap, and the
SFF-8087 breakout cables are also less of a pain to route around cases than
individual SATA cables.