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Thread: EXT4-fs (sda6): Unaligned AIO/DIO on inode N by VirtualBox; performance will be poor

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  1. #1
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    EXT4-fs (sda6): Unaligned AIO/DIO on inode N by VirtualBox; performance will be poor

    Well I've seen the source code that produces this log entry, but I'm none the wiser as to whether or not this is a problem that needs addressing, or how.

    Anyone have any information on this?

  2. #2
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    Re: EXT4-fs (sda6): Unaligned AIO/DIO on inode N by VirtualBox; performance will be p

    I've not looked at the source code, but my hunch is it's related to issues of sector alignment on Advanced Format drives, RAID arrays, and SSDs. On such disks, if the data structures in the virtual disk don't align properly to the underlying physical sectors (or other data structures, such as RAID stripes), then performance can suffer.

    I'm afraid I've not investigated this issue with respect to virtual disks in any detail, though, and I don't have specific suggestions for how to fix or work around the problem. AFAIK, though, it's only likely to be a problem on one of the devices that's susceptible to the issue in the first place, so depending on your disk hardware, it might be a non-issue. If you're not sure about your disk hardware on this score, please post back with details of what you've got.
    If I've suggested a solution to a problem and you're not the original poster, do not try my solution! Problems can seem similar but be different, and a good solution to one problem can make another worse. Post a new thread with your problem details.

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    Re: EXT4-fs (sda6): Unaligned AIO/DIO on inode N by VirtualBox; performance will be p

    Quote Originally Posted by srs5694 View Post
    I've not looked at the source code, but my hunch is it's related to issues of sector alignment on Advanced Format drives, RAID arrays, and SSDs. On such disks, if the data structures in the virtual disk don't align properly to the underlying physical sectors (or other data structures, such as RAID stripes), then performance can suffer.

    I'm afraid I've not investigated this issue with respect to virtual disks in any detail, though, and I don't have specific suggestions for how to fix or work around the problem. AFAIK, though, it's only likely to be a problem on one of the devices that's susceptible to the issue in the first place, so depending on your disk hardware, it might be a non-issue. If you're not sure about your disk hardware on this score, please post back with details of what you've got.
    I have an ext4 partition (on a SATA HD) containing two VirtualBox drives formatted to NTFS. The log warning seems to imply that I'm getting sub-optimal performance when those "drives" are being written to because of the underlying ext4 partition. So far, I've not noticed any real drop in performance, but if I start to (I've only had Natty installed for a week), then I may have to change the ext4 to an ext3 partition - with the possibility of first attempting to mount the ext4 partition as ext3 instead, which I believe is possible.
    Last edited by imbjr; May 6th, 2011 at 01:22 PM. Reason: Added type of drive

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    Re: EXT4-fs (sda6): Unaligned AIO/DIO on inode N by VirtualBox; performance will be p

    Here's some extra data:

    Disk /dev/sda: 320.1 GB, 320072933376 bytes
    255 heads, 63 sectors/track, 38913 cylinders
    Units = cylinders of 16065 * 512 = 8225280 bytes
    Sector size (logical/physical): 512 bytes / 512 bytes
    I/O size (minimum/optimal): 512 bytes / 512 bytes
    Disk identifier: 0x000adaa6

    Device Boot Start End Blocks Id System
    /dev/sda1 * 1 38914 312568833 5 Extended
    /dev/sda5 1 523 4194304 82 Linux swap / Solaris
    /dev/sda6 523 5092 36700160 83 Linux
    /dev/sda7 5092 38914 271672320 83 Linux
    Note:
    . There's an unallocated section before /dev/sda1 of 2Mb in size.
    . sda6 and sda7 are formatted to ext4.

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    Re: EXT4-fs (sda6): Unaligned AIO/DIO on inode N by VirtualBox; performance will be p

    I have the same issue with a RHEL VM. I've cloned the virtual HD and am running off a different image that when the error started appearing but it still occurs. Now I see it happen sometimes, but not all the time, when I start up the RHEL VM. I'm stumped.

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    Re: EXT4-fs (sda6): Unaligned AIO/DIO on inode N by VirtualBox; performance will be p

    Quote Originally Posted by imbjr View Post
    I have an ext4 partition (on a SATA HD) containing two VirtualBox drives formatted to NTFS. The log warning seems to imply that I'm getting sub-optimal performance when those "drives" are being written to because of the underlying ext4 partition.
    That's not my interpretation -- but I didn't write the code in question, either. I certainly know of no reason why ext4fs would produce worse results than ext3fs in this sort of situation.

    Your fdisk output is uninformative. Even when dealing with conventional alignment issues on real hard disks, you need sector-precise values, not the much sloppier cylinder values produced by "fdisk -l". Please read the article in the link in my first post. In a virtual disk, you've also got to be concerned with how the virtual disk lays out its data, and that's something I don't know much about.
    If I've suggested a solution to a problem and you're not the original poster, do not try my solution! Problems can seem similar but be different, and a good solution to one problem can make another worse. Post a new thread with your problem details.

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    Re: EXT4-fs (sda6): Unaligned AIO/DIO on inode N by VirtualBox; performance will be p

    Quote Originally Posted by srs5694 View Post
    That's not my interpretation -- but I didn't write the code in question, either. I certainly know of no reason why ext4fs would produce worse results than ext3fs in this sort of situation.

    Your fdisk output is uninformative. Even when dealing with conventional alignment issues on real hard disks, you need sector-precise values, not the much sloppier cylinder values produced by "fdisk -l". Please read the article in the link in my first post. In a virtual disk, you've also got to be concerned with how the virtual disk lays out its data, and that's something I don't know much about.
    Well according the data sheet for my WD3200KS-75P disk, there are 512 bytes per sector, as per the fdisk info above.

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    Re: EXT4-fs (sda6): Unaligned AIO/DIO on inode N by VirtualBox; performance will be p

    In that case, you shouldn't worry about it, at least not if my interpretation of the message is correct. If there are alignment issues within a filesystem that are unrelated to the logical vs. physical sector size issues, RAID issues, or SSD issues, then there may be something to worry about.

    You might try digging into the VirtualBox documentation or even contact a developer about it if you want to be 100% sure.
    If I've suggested a solution to a problem and you're not the original poster, do not try my solution! Problems can seem similar but be different, and a good solution to one problem can make another worse. Post a new thread with your problem details.

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