Re: sd_resume redundant? [was: [PATCH] libata: implement ata_wait_after_reset()]

[Tejun Heo <htejun@gmail.com>]

Indan Zupancic wrote:
>> Can you try to measure with sd_resume in place?
> 
> [    2.173366] sd 0:0:0:0: [sda] Starting disk
> [    2.475422] ata2: SATA link down (SStatus 0 SControl 310)
> [    5.478403] ata1: SATA link up 1.5 Gbps (SStatus 113 SControl 310)
> [    5.481928] ata1.00: ata_hpa_resize 1: sectors = 234441648, hpa_sectors = 234441648
> [    5.485904] ata1.00: ata_hpa_resize 1: sectors = 234441648, hpa_sectors = 234441648
> [    5.485913] ata1.00: configured for UDMA/100
> [    5.505109] sd 0:0:0:0: [sda] 234441648 512-byte hardware sectors (120034 MB)
> [    5.505461] sd 0:0:0:0: [sda] Write Protect is off
> [    5.505465] sd 0:0:0:0: [sda] Mode Sense: 00 3a 00 00
> [    5.505612] sd 0:0:0:0: [sda] Write cache: enabled, read cache: enabled, doesn't support DPO or
> FUA
> ...
> [    6.157259] Restarting tasks ... done.
> 
> 
> And with echo 0 > /sys/class/scsi_disk/0\:0\:0\:0/manage_start_stop:
> 
> [    2.476476] ata2: SATA link down (SStatus 0 SControl 310)
> ...
> [    2.825479] Restarting tasks ... done.
> ...
> [    5.022076] ata1: SATA link up 1.5 Gbps (SStatus 113 SControl 310)
> [    5.025605] ata1.00: ata_hpa_resize 1: sectors = 234441648, hpa_sectors = 234441648
> [    5.028594] ata1.00: ata_hpa_resize 1: sectors = 234441648, hpa_sectors = 234441648
> [    5.028606] ata1.00: configured for UDMA/100
> [    5.028720] sd 0:0:0:0: [sda] 234441648 512-byte hardware sectors (120034 MB)
> [    5.028767] sd 0:0:0:0: [sda] Write Protect is off
> [    5.028773] sd 0:0:0:0: [sda] Mode Sense: 00 3a 00 00
> [    5.028831] sd 0:0:0:0: [sda] Write cache: enabled, read cache: enabled, doesn't support DPO or
> FUA
> 
> So over all it takes half a second longer to detect the disk, but
> because everything waits on it, it takes more than three seconds
> longer to resume.

Eeeek.  Extra three secs doesn't sound too hot.  :-(

> Setting manage_start_stop to 0 fixes it and is good enough for me, I
> didn't notice anything bad yet because of the unmanaged
> stop. Implementing background spin up will fix it too.

Just commenting out sd_resume() would be a better solution for your
case tho.

>>> Everything seems to work fine without sd_resume(), so why is it needed?
>> Because not all disks spin up without being told to do so and like it or
>> not spinning disks up on resume is the default behavior.  As I wrote in
>> the other reply, it would be worthwhile to make it configurable.
> 
> Not even after they receive a read command? Ugh.

After receiving a command which requires media access, they do.  What
I was saying is that the current default behavior is to spin up all
devices on resume and part of that is achieved by sd_resume().

Hmmm... skipping START_STOP during sd_resume() actually is a pretty
good solution for ATA devices.  I'll think about it.

Thanks.

-- 
tejun