On 4/2/2012 2:04 AM, David Brown wrote: > On 02/04/2012 07:43, Stan Hoeppner wrote: >> On 4/1/2012 5:33 AM, David Brown wrote: >> >>> For an application like this, it would probably make sense to put the >>> xfs log (and the mdraid bitmap file, if you are using one) on a separate >>> disk - perhaps a small SSD. >> >> XFS only journals metadata changes. Thus an external journal is not >> needed here as there is no metadata in the workload. >> > > Won't there still be metadata changes as each file is closed and a new > one started, Yes, creating one file requires one journal operation consisting of a few dozen to a few hundred bytes of journal data. With delaylog this will very likely be consolidated into a single IO to the journal, or a handful of IOs. With the workload in question the number of journal IOs is trivial. > or as more space is allocated to the files being written (I Allocation occurs once when the file is created. > know xfs has delayed allocation and other tricks to minimise this, but > it can't be eliminated entirely). Each time the metadata or log needs > written, you'll get a big discontinuity in the writing as the disks > heads need to jump around (especially for RAID5!). The ratio of file IOs to metadata IOs in this workload is well north of millions to one. Again, the metadata IOPS in this workload are trivial. RAID level has no bearing on metadata IOPS here. Though it does for the actual workload, as I already discussed in detail. > It would make sense to have a separate disk (again, SSD is logical) for > the OS and other files anyway, keeping the big array for the data. > Putting the xfs log there too would surely be a small but helpful step? No it doesn't make sense. See above. Additionally, an external journal can only be used if the journal device is behind battery backed RAID cache. Due to a design issue, XFS cannot perform barrier operations on an external journal device, only to an internal journal. Without barriers or BBWC you have the opportunity for constant filesystem corruption. The OP doesn't have a BBWC RAID controller, nor any RAID controller at all. So an external journal isn't an option here anyway. -- Stan -- 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
