geom write cache handling

[krichy at tvnetwork.hu]

Fortunately I have my drives attached to a SAS controller, so I could add 
quirks for them. They are also attached.

Regards,
Kojedzinszky Richard
Euronet Magyarorszag Informatika Zrt.

On Thu, 6 Feb 2014, Richard Kojedzinszky wrote:

> Date: Thu, 6 Feb 2014 15:36:54 +0100 (CET)
> From: Richard Kojedzinszky <krichy at cflinux.hu>
> To: freebsd-fs at freebsd.org
> Subject: geom write cache handling
> 
> Dear fs team,
>
> I own an STEC SSD, which I use for ZFS SLOG. The device is broken in that if 
> it gets a SCSI synchronize cache, it does something which is very slow. 
> Actually with it in FreeBSD the reachable sync IOPS is around 100. When the 
> device has write cache disabled, there is no need to send the SCSI 
> synchronize cache commands to it, and without them I can reach 1400 IOPS. The 
> device itself have power-loss-protection, so I risk no data corruption at 
> all.
>
> By the way, linux behave the same, if either ata or scsi disks have their 
> write cache turned off, it even does not send the command to the drive.
>
> Also, I have an Intel S3700, which is much faster, but have similar symptons. 
> The drive handles synchronize cache commands much faster than STEC, maybe it 
> is implemented in the drive as a simple NOP, but as for 4K sync writes a sync 
> cache follows, it effectively halves the IOPS. I have it attached to a SATA2 
> controller only, and with WCE disabled, and sending the sync cache to it I 
> can reach around 4500 IOPS, while disabling the sync cache it can reach >9000 
> IOPS.
>
> I've attached a very simple patch for the ata layer, but I dont know how to 
> implement it for the scsi subsystem also.
>
> Regards,
>
> Kojedzinszky Richard
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