Re: [Qemu-devel] XP install cores with SCSI LSI 53C895A disks

[Gerhard Wiesinger <lists@wiesinger.com>]

On Thu, 8 Mar 2012, Gerd Hoffmann wrote:

> On 03/07/12 20:58, Gerhard Wiesinger wrote:
>> On Wed, 7 Mar 2012, Brian Jackson wrote:
>>> I think most people trying to use qemu for anything useful have given
>>> up on if=scsi. Some distros even disable support because they don't
>>> want to QA it. That should be a decent sign that you may want to avoid
>>> it.
>>
>> OK, but SAS (Serial attached SCSI) is technology in the area of storage
>> interface technology where all big storage vendors see future (e.g. they
>> give up: FC and SATA drives, SATA drives are replaced by MDL SATA drives
>> (SATA 7200RPM drives with SAS interface)).
>
> The problem isn't scsi.  The problem is the lsi adapter.  Problem #1 is
> the hardware design which makes it hard to emulate it correctly and #2
> that you need a non-redistributable rom file to boot from it.

Advantages of LSI 53C895A over others:
1.) OS support is great, even for legacy systems: DOS, Win 3.1, Win95, NT 
4, W2K, XP, Vista, Win7, Linux, etc. I don't know any adapter with such 
wide range of OS support. Also tested up to 2TB of LUNs.
2.) OS support out of the box without additional drivers for a lot of 
newer OSes
3.) Migration from VMWare SCSI is easy
4.) Works well here for about 1 year for legacy VMs for DOS and NT 4
5.) BIOS geometry translation is correct, also according to 
partition table (doesn't work correctly on MEGASAS,
INT13 geometry interrupts are not correct, already reported to 
Hannes)
7.) LSI SCSI DMA technology is fast. I'm getting /dev/null performance 
over 500MB/s ..., optimized for parallel IOPS, etc.
8.) Faster ROM init than MEGASAS

Disadvantages:
1.) ROM non distributable
2.) non complete implementation
3.) 2TB limit
4.) Slower ROM INIT than without any Option ROM

Megasas also needs a ROM rom to boot from it.
Not to forget iSCSI which is also current/future SCSI technology ...

Even if it is hard but I think the goal of qemu should be to implement all 
supported hardware pieces as good as possible (e.g. LSI 53C895A). I know 
it is not an easy task (see my thread about rtl8139) but I think together 
we can manage it.

Are there any LSI 53C895A known bugs or incomplete implementation issues 
which are known? 
Any hints on the core dump?

>> Therefore I don't understand why distros are giving up SAS which is also
>> SCSI (of course old legacy SCSI is understandable).
>
> Nobody gives up on scsi.  See virtio-scsi merged recently.  There also
> is megasas aiming for merge (which shares the boot issue with lsi though).

Disadvantage of virtio-scsi is that drivers are needed.
Is INT13h supported for legacy OS and to boot? Future? But as far as I saw 
implemented in seabios, right?
What BIOS translation is used? Buslogic? LSI logic? Partition guessing?

BTW: I've found out how LSI logic BIOS geometry translation works with 
(guessing) and without existing partition table. That might be an option
for Seabios to implement.
@Kevin: What do you think?

Ciao,
Gerhard

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