On Wed, May 30, 2012 at 4:32 PM, James Bottomley <James.Bottomley@xxxxxxxxxxxxxxxxxxxxx> wrote: > On Wed, 2012-05-30 at 11:26 -0700, Dan Williams wrote: >> On Mon, May 28, 2012 at 5:07 AM, James Bottomley >> <James.Bottomley@xxxxxxxxxxxxxxxxxxxxx> wrote: >> > On Mon, 2012-05-28 at 10:00 +0000, maximilian attems wrote: >> >> On Sun, May 27, 2012 at 10:13:46AM +0100, James Bottomley wrote: >> >> > scsi_wait_scan was introduced with asynchronous host scanning as a hack >> >> > for distributions that weren't using proper udev based wait for root to >> >> > appear in their initramfs scripts. In 2.6.30 Commit >> >> >> >> > c751085943362143f84346d274e0011419c84202 >> >> > Author: Rafael J. Wysocki <rjw@xxxxxxx> >> >> > Date: Sun Apr 12 20:06:56 2009 +0200 >> >> > >> >> > PM/Hibernate: Wait for SCSI devices scan to complete during resume >> >> > >> >> > Actually broke scsi_wait_scan because it renders >> >> > scsi_complete_async_scans() a nop for modular SCSI if you include >> >> > scsi_scans.h (which this module does). >> >> > >> >> > The lack of bug reports is sufficient proof that this module is no >> >> > longer used. >> >> >> >> We do use it in initramfs-tools. >> >> >> >> There is quite a number of bug reports moaning about having to boot with >> >> `scsi_mod.scan=sync'. I didn't pass them on, because I didn't knew that >> >> the module itself got broken, for example: >> >> http://bugs.debian.org/616689 >> > >> > OK, so what these bugs show is the breakage ... basically scsi_wait_scan >> > isn't really waiting for the scans to complete. I can fix it in stable >> > so you can close your bug reports, but if I do, can you also transition >> > away from using it so I can remove it in 3.5? >> >> Is there some other method whereby userspace can sync all driver >> probing actions? > > No, but then there never really was. The theory is you know all the > disks you need (/ /usr and so on) and you just wait for them to appear > before mounting them and proceeding with boot. > >> We won't need scsi_complete_async_scans() after: >> >> http://marc.info/?l=linux-scsi&m=133840132007532&w=2 >> >> ...but won't initramfs environments still need a way to trigger >> wait_for_device_probe()? Something like echo "flush" > >> /sys/devices/async_probe. and maybe reading that file indicates if >> some async probing is still in-flight? > > Why? The job of an initramfs is to mount root. All it has to do is > wait for root to appear via udev and then proceed. The whole reason for > doing stuff async initially was to speed boot, so probing can still be > ongoing even after the initrd exits. > > If you think about it, most modern fabrics are hot plug. Just because > the initial scan has completed there's no guarantee that all the devices > have appeared yet. Fine for single device root, but what about raid and degraded assembly? Last time I checked scsi_wait_scan was still being used by dracut in the case where it decides to stop waiting for all raid members to appear. It's a "last call" before proceeding with degraded assembly. If you immediately assemble and mount root as soon as the root device could be started it will almost always be a degraded array. Sure the initramfs can just timeout arrival, but at a minimum that timeout should be "load module + flush scanning". Without a flush mechanism it's just a shot in the dark what that minimum timeout should be. If ata error recovery is kicking in and needs 10s of seconds to recover a drive I'd want my initramfs to wait for that process to quiesce before timing out and moving on. -- Dan -- To unsubscribe from this list: send the line "unsubscribe linux-scsi" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
