nice, i don´t know if it´s a problem of single thread i think it´s a problem about async read command being executed in parallel i post again at https://bbs.archlinux.org/viewtopic.php?pid=887345 please see the history at the end of page i´m talking about a disk with 5000rpm and a disk with 7000rpm i think we can optimize mirror read algorithm and it´s not very hard for same speed hard disk, near mirror is good for same speed solid state, round robin is good for anyone, time based is good diferences? hard disk: time to position head is high, time to read can be small solid state: time to position is small, time to read is small (some ssd are old, and have small read rate) nbd: time based on server hard/solid disk, and network time, but don´t think in nbd yet 2011/2/2 Keld Jørn Simonsen <keld@xxxxxxxxxx>: > Hmm, Roberto, I think we are close to theoretical maximum with > some of the raid1/raid10 stuff already. and my nose tells me > that we can gain more by minimizing CPU usage. > Or maybe using some threading for raid modules - they > all run single-threaded. > > Best regards > keld > > > On Wed, Feb 02, 2011 at 06:28:27PM -0200, Roberto Spadim wrote: >> before, this thread i put at this page: >> https://bbs.archlinux.org/viewtopic.php?pid=887267 >> to make this mail list with less emails >> >> 2011/2/2 Keld Jørn Simonsen <keld@xxxxxxxxxx>: >> > Hmm, Roberto, where are the gains? >> >> it?s dificult to talk... NCQ and linux scheduler don?t help a mirror, >> they help a single device >> a new scheduler for mirrors can be done (round robin, closest head, others) >> >> > I think it is hard to make raid1 better than it is today. >> i don?t think, since head, is just for hard disk (rotational) not for >> solid state disks, let?s not talk about ssd, just hard disk? a raid >> with 5000rpm and 10000rpm disk, we will have better i/o read with >> 10000rpm ? we don?t know the model of i/o for that device, but >> probally will be faster, but when it?s busy we could use 5000rpm... >> that?s the point, just closest head don?t help, we need know what?s >> the queue (list of i/o being processed) and the time to read the >> current i/o >> >> > Normally the driver orders the reads to minimize head movement >> > and loss with rotation latency. Where can we improve that? >> >> no way to improve it, it?s very good! but per hard disk, not per mirror >> but since we know it?s busy we can use another mirror (another disk >> with same information), that?s what i want >> >> > Also, what about conflicts with the elevator algorithm? >> elevator are based on model of disk, think disk as: linux elevator + >> NCQ + disks, the sum of three infomration give us time based >> infomrations to select best device >> maybe making complex code (per elevator) we could know the time spent >> to execute it, but it?s a lot of work, >> for the first model, lets think about parameters of our model (linux >> elevator + ncq + disks) >> a second version we could implement elevator algorithm time >> calculation (network block device NBD, have a elevator? at server side >> + tcp/ip stack at client and server side, right?) >> >> > There are several scheduling algorithms available, and each has >> > its merits. Will your new scheme work against these? >> > Or is your new scheme just another scheduling algorithm? >> >> it?s a scheduling for mirrors >> round balance is a algorithm for mirror >> closest head is a algorithm for mirror >> my 'new' algorith will be for mirror (if anyone help me coding for >> linux kernel hehehe, i didn?t coded for linux kernel yet, just for >> user space) >> >> noop, deadline, cfq isn?t for mirror, these are for raid0 problem >> (linear, stripe if you hard disk have more then one head on your hard >> disk) >> >> > I think I learned that scheduling is per drive, not per file system. >> yes, you learned right! =) >> /dev/md0 (raid1) is a device with scheduling (closest head,round robin) >> /dev/sda is a device with scheduling (noop, deadline, cfq, others) >> /dev/sda1 is a device with scheduling (it send all i/o directly to /dev/sda) >> >> the new algorithm is just for mirrors (raid1), i dont remeber about >> raid5,6 if they are mirror based too, if yes they could be optimized >> with this algorithm too >> >> raid0 don?t have mirrors, but information is per device striped (not >> for linear), that?s why it can be faster... can make parallel reads >> >> with closest head we can?t use best disk, we can use a single disk all >> time if it?s head closer, maybe it?s not the fastest disk (that?s why >> we implent the write-mostly, we don?t make they usable for read, just >> for write or when mirror fail, but it?s not perfect for speed, a >> better algorithm can be made, for identical disks, a round robin work >> well, better than closest head if it?s a solid state disk) >> ok on a high load, maybe closest mirror is better than this algorithm? >> yes, if you just use hard disk, if you mix hard disk+solid >> state+network block device +floppy disks+any other device, you don?t >> have the best algorithm for i/o over mirrors >> >> >> > and is it reading or writing or both? Normally we are dependant on the >> > reading, as we cannot process data before we have read them. >> > OTOH writing is less time critical, as nobody is waiting for it. >> it must be implemented on write and read, write for just time >> calculations, read for select the best mirror >> for write we must write on all mirrors (sync write is better, async >> isn?t power fail safe) >> >> > Or is it maximum thruput you want? >> > Or a mix, given some restraints? >> it?s the maximum performace = what?s the better strategy to spent less >> time to execute current i/o, based on time to access disk, time to >> read bytes, time to wait others i/o being executed >> >> that?s for mirror select, not for disks i/o >> for disks we can use noop, deadline, cfq scheduller (for disks) >> tcp/ip tweaks for network block device >> >> a model identification must execute to tell the mirror select >> algorithm what?s the model of each device >> model: time to read X bytes, time to move head, time to start a read, >> time to write, time time time per byte per kb per units >> calcule time and select the minimal value calculated as the device >> (mirror) to execute our read >> >> >> > >> > best regards >> > keld >> >> thanks keld >> >> sorry if i make email list very big >> >> >> >> -- >> Roberto Spadim >> Spadim Technology / SPAEmpresarial > -- > To unsubscribe from this list: send the line "unsubscribe linux-raid" in > the body of a message to majordomo@xxxxxxxxxxxxxxx > 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@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
