Re: SSD - TRIM command

[Piergiorgio Sartor <piergiorgio.sartor@nexgo.de>]

> yeah =)
> a question...
> if i send a TRIM to a sector
> if i read from it
> what i have?
> 0x00000000000000000000000000000000000 ?
> if yes, we could translate TRIM to WRITE on devices without TRIM (hard disks)
> just to have the same READ information

It seems the 0x0 is not a standard. Return values
seem to be quite undefined, even if 0x0 *might*
be common.

Second, why do you want to emulate the 0x0 thing?

I do not see the point of writing zero on a device
which do not support TRIM. Just do nothing seems a
better choice, even in mixed environment.

bye,

pg
 
> 2011/2/9 Piergiorgio Sartor <piergiorgio.sartor@nexgo.de>:
> >> it´s just a discussion, right? no implementation yet, right?
> >
> > Of course...
> >
> >> what i think....
> >> if device accept TRIM, we can use TRIM.
> >> if not, we must translate TRIM to something similar (maybe many WRITES
> >> ?), and when we READ from disk we get the same information
> >
> > TRIM is not about writing at all. TRIM tells the
> > device that the addressed block is not anymore used,
> > so it (the SSD) can do whatever it wants with it.
> >
> > The only software layer having the same "knowledge"
> > is the filesystem, the other layers, do not have
> > any decisional power about the block allocation.
> > Except for metadata, of course.
> >
> > So, IMHO, a software TRIM can only be in the FS.
> >
> > bye,
> >
> > pg
> >
> >> the translation coulbe be done by kernel (not md) maybe options on
> >> libata, nbd device....
> >> other option is do it with md, internal (md) TRIM translate function
> >>
> >> who send trim?
> >> internal md information: md can generate it (if necessary, maybe it´s
> >> not...) for parity disks (not data disks)
> >> filesystem/or another upper layer program (database with direct device
> >> access), we could accept TRIM from filesystem/database, and send it to
> >> disks/mirrors, when necessary translate it (internal or kernel
> >> translate function)
> >>
> >>
> >> 2011/2/9 Piergiorgio Sartor <piergiorgio.sartor@nexgo.de>:
> >> > On Wed, Feb 09, 2011 at 04:30:15PM -0200, Roberto Spadim wrote:
> >> >> nice =)
> >> >> but check that parity block is a raid information, not a filesystem information
> >> >> for raid we could implement trim when possible (like swap)
> >> >> and implement a trim that we receive from filesystem, and send to all
> >> >> disks (if it´s a raid1 with mirrors, we should sent to all mirrors)
> >> >
> >> > To all disk also in case of RAID-5?
> >> >
> >> > What if the TRIM belongs only to a single SDD block
> >> > belonging to a single chunk of a stripe?
> >> > That is a *single* SSD of the RAID-5.
> >> >
> >> > Should md re-read the block and re-write (not TRIM)
> >> > the parity?
> >> >
> >> > I think anything that has to do with checking &
> >> > repairing must be carefully considered...
> >> >
> >> > bye,
> >> >
> >> > pg
> >> >
> >> >> i don´t know what trim do very well, but i think it´s a very big write
> >> >> with only some bits for example:
> >> >> set sector1='00000000000000000000000000000000000000000000000000'
> >> >> could be replace by:
> >> >> trim sector1
> >> >> it´s faster for sata communication, and it´s a good information for
> >> >> hard disk (it can put a single '0' at the start of the sector and know
> >> >> that all sector is 0, if it try to read any information it can use
> >> >> internal memory (don´t read hard disk), if a write is done it should
> >> >> write 0000 to bits, and after after the write operation, but it´s
> >> >> internal function of hard disk/ssd, not a problem of md raid... md
> >> >> raid should need know how to optimize and use it =] )
> >> >>
> >> >> 2011/2/9 Piergiorgio Sartor <piergiorgio.sartor@nexgo.de>:
> >> >> >> ext4 send trim commands to device (disk/md raid/nbd)
> >> >> >> kernel swap send this commands (when possible) to device too
> >> >> >> for internal raid5 parity disk this could be done by md, for data
> >> >> >> disks this should be done by ext4
> >> >> >
> >> >> > That's an interesting point.
> >> >> >
> >> >> > On which basis should a parity "block" get a TRIM?
> >> >> >
> >> >> > If you ask me, I think the complete TRIM story is, at
> >> >> > best, a temporary patch.
> >> >> >
> >> >> > IMHO the wear levelling should be handled by the filesystem
> >> >> > and, with awarness of this, by the underlining device drivers.
> >> >> > Reason is that the FS knows better what's going on with the
> >> >> > blocks and what will happen.
> >> >> >
> >> >> > bye,
> >> >> >
> >> >> > pg
> >> >> >
> >> >> >>
> >> >> >> the other question... about resync with only write what is different
> >> >> >> this is very good since write and read speed can be different for ssd
> >> >> >> (hd don´t have this 'problem')
> >> >> >> but i´m sure that just write what is diff is better than write all
> >> >> >> (ssd life will be bigger, hd maybe... i think that will be bigger too)
> >> >> >>
> >> >> >>
> >> >> >> 2011/2/9 Eric D. Mudama <edmudama@bounceswoosh.org>:
> >> >> >> > On Wed, Feb  9 at 11:28, Scott E. Armitage wrote:
> >> >> >> >>
> >> >> >> >> Who sends this command? If md can assume that determinate mode is
> >> >> >> >> always set, then RAID 1 at least would remain consistent. For RAID 5,
> >> >> >> >> consistency of the parity information depends on the determinate
> >> >> >> >> pattern used and the number of disks. If you used determinate
> >> >> >> >> all-zero, then parity information would always be consistent, but this
> >> >> >> >> is probably not preferable since every TRIM command would incur an
> >> >> >> >> extra write for each bit in each page of the block.
> >> >> >> >
> >> >> >> > True, and there are several solutions.  Maybe track space used via
> >> >> >> > some mechanism, such that when you trim you're only trimming the
> >> >> >> > entire stripe width so no parity is required for the trimmed regions.
> >> >> >> > Or, trust the drive's wear leveling and endurance rating, combined
> >> >> >> > with SMART data, to indicate when you need to replace the device
> >> >> >> > preemptive to eventual failure.
> >> >> >> >
> >> >> >> > It's not an unsolvable issue.  If the RAID5 used distributed parity,
> >> >> >> > you could expect wear leveling to wear all the devices evenly, since
> >> >> >> > on average, the # of writes to all devices will be the same.  Only a
> >> >> >> > RAID4 setup would see a lopsided amount of writes to a single device.
> >> >> >> >
> >> >> >> > --eric
> >> >> >> >
> >> >> >> > --
> >> >> >> > Eric D. Mudama
> >> >> >> > edmudama@bounceswoosh.org
> >> >> >> >
> >> >> >> > --
> >> >> >> > To unsubscribe from this list: send the line "unsubscribe linux-raid" in
> >> >> >> > the body of a message to majordomo@vger.kernel.org
> >> >> >> > More majordomo info at  http://vger.kernel.org/majordomo-info.html
> >> >> >> >
> >> >> >>
> >> >> >>
> >> >> >>
> >> >> >> --
> >> >> >> Roberto Spadim
> >> >> >> Spadim Technology / SPAEmpresarial
> >> >> >> --
> >> >> >> To unsubscribe from this list: send the line "unsubscribe linux-raid" in
> >> >> >> the body of a message to majordomo@vger.kernel.org
> >> >> >> More majordomo info at  http://vger.kernel.org/majordomo-info.html
> >> >> >
> >> >> > --
> >> >> >
> >> >> > piergiorgio
> >> >> > --
> >> >> > To unsubscribe from this list: send the line "unsubscribe linux-raid" in
> >> >> > the body of a message to majordomo@vger.kernel.org
> >> >> > More majordomo info at  http://vger.kernel.org/majordomo-info.html
> >> >> >
> >> >>
> >> >>
> >> >>
> >> >> --
> >> >> Roberto Spadim
> >> >> Spadim Technology / SPAEmpresarial
> >> >> --
> >> >> To unsubscribe from this list: send the line "unsubscribe linux-raid" in
> >> >> the body of a message to majordomo@vger.kernel.org
> >> >> More majordomo info at  http://vger.kernel.org/majordomo-info.html
> >> >
> >> > --
> >> >
> >> > piergiorgio
> >> > --
> >> > To unsubscribe from this list: send the line "unsubscribe linux-raid" in
> >> > the body of a message to majordomo@vger.kernel.org
> >> > More majordomo info at  http://vger.kernel.org/majordomo-info.html
> >> >
> >>
> >>
> >>
> >> --
> >> Roberto Spadim
> >> Spadim Technology / SPAEmpresarial
> >
> > --
> >
> > piergiorgio
> > --
> > To unsubscribe from this list: send the line "unsubscribe linux-raid" in
> > the body of a message to majordomo@vger.kernel.org
> > More majordomo info at  http://vger.kernel.org/majordomo-info.html
> >
> 
> 
> 
> -- 
> Roberto Spadim
> Spadim Technology / SPAEmpresarial

-- 

piergiorgio
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