[mythtv-users] Mythtv 24.1 - MythFillDatabase filling the database
Michael T. Dean
mtdean at thirdcontact.com
Thu Mar 1 19:41:59 UTC 2012
On 03/01/2012 09:48 AM, Trevor Burns wrote:
> The database is currently on an EXT4 file system. Looking at fstab (see
> below), I am not seeing any indication of barriers for the mounts. One item
> I forgot to include, the two 500G drives are "linked" as a LVM volume group.
> /dev/mapper/vg_pvr2-lv_root / ext4 defaults
> 1 1
> UUID=3a613e91-3559-4f4e-8344-0a5115b08c4b /boot ext4
> defaults 1 2
> /dev/mapper/vg_pvr2-lv_swap swap swap defaults
> 0 0
> tmpfs /dev/shm tmpfs defaults 0 0
> devpts /dev/pts devpts gid=5,mode=620 0 0
> sysfs /sys sysfs defaults 0 0
> proc /proc proc defaults 0 0
Since barriers are enabled by default on ext4, and yours is set to
defaults, you have them enabled.
You can change this by specifying a mount option of barrier=0.
I would say it's worth testing with barrier=0 set. If mythfilldatabase
works without barriers enabled, you can then choose whether the benefits
outweigh the danger of running without barriers enabled.
As a quick summary, barriers cause sync operations (something MySQL--and
any other database--requests relatively frequently) to block until the
data is actually on the HDD platter (=safe on non-volatile storage).
Without barriers, a sync operation will return (allowing the program,
MySQL, to continue doing its job) as soon as the data being written is
in the HDD cache (volatile RAM buffer in the HDD)--meaning that if you
lose power, you could lose data(/will lose all data in the cache),
potentially even critical file system data that could corrupt the file
system. These losses are even possible with ext3 and ext4--which are
journaling file systems--if barriers aren't enabled. This is why the
default for ext4 (and, at least in some distros, the default for ext3 on
new kernels/configurations) was changed to enable barriers.
Again, I'm not recommending running without barriers enabled. I'm only
recommending a quick test without barriers enabled to see if the
slowness is causing the issue for mythfilldatabase. If so, you will
have to decide what to do to mitigate the issue.
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