Re: [Qemu-devel] [PATCH v4 2/2] qemu-ga: sample fsfreeze hooks

[Tomoki Sekiyama <tomoki.sekiyama.qu@hitachi.com>]

On 2012/11/23 1:03, Luiz Capitulino wrote:
> On Thu, 22 Nov 2012 11:15:49 +0900
> Tomoki Sekiyama <tomoki.sekiyama.qu@hitachi.com> wrote:
> 
>> Adds sample hook scripts for --fsfreeze-hook option of qemu-ga.
>>   - fsfreeze-hook : execute scripts in fsfreeze-hook.d/
>>   - fsfreeze-hook.d.sample/mysql-flush.sh : quiesce MySQL before snapshot
<snip>
>> +# Iterate executables in directory "fsfreeze-hook.d" with the specified args
>> +[ ! -d "$FSFREEZE_D" ] && exit 1
>> +for file in "$FSFREEZE_D"/* ; do
>> +    is_ignored_file "$file" && continue
>> +    [ -x "$file" ] || continue
>> +    echo "$(date): execute $file $@" >>$LOGFILE
>> +    "$file" "$@" >>$LOGFILE 2>&1
>> +    STATUS=$?
>> +    echo "$(date): $file finished with status=$STATUS" >>$LOGFILE
>> +done
> 
> execute_fsfreeze_hook() will fail the freeze process if this script fails. Two
> comments:
> 
>  1. Do we want to fail the freeze process if one of the sub-scripts fails?
>     If yes, then we have to exit 1 in the first failure

I originally thought the hooks are optional; even they failed, filesystem-level
consistency are still kept in the snapshot.
However, if we are going to fail fsfreeze process by one of sub-scripts'
failure, we also need to notify scripts which already succeeded to pre-freeze
to thaw (or abort) freezing, before exit 1 here.

>  2. The exit status of the script will echo's exit status. I doubt we want that

Do you mean $STATUS (not $status) is specialized in some shell environments?

>> diff --git a/docs/qemu-guest-agent/fsfreeze-hook.d.sample/mysql-flush.sh b/docs/qemu-guest-agent/fsfreeze-hook.d.sample/mysql-flush.sh
>> new file mode 100755
>> index 0000000..e6d7998
>> --- /dev/null
>> +++ b/docs/qemu-guest-agent/fsfreeze-hook.d.sample/mysql-flush.sh
>> @@ -0,0 +1,47 @@
>> +#!/bin/sh
>> +
>> +# Flush MySQL tables to the disk before the filesystem is freezed.
>> +# At the same time, this keeps a read lock while the filesystem is freezed
>> +# in order to avoid write accesses by the other clients.
>> +
>> +MYSQL="mysql -uroot" #"-prootpassword"
>> +FIFO=/tmp/mysql-flush.fifo
>> +
>> +flush_and_wait() {
>> +    printf "FLUSH TABLES WITH READ LOCK \\G\n"
>> +    read < $FIFO
>> +    printf "UNLOCK TABLES \\G\n"
>> +}
>> +
>> +case "$1" in
>> +    freeze)
>> +        mkfifo $FIFO || exit 1
>> +        flush_and_wait | $MYSQL &
> 
> Honest question: what happens if I don't have mysql installed?

Ah OK, availability of mysql should be checked in advance.

Thanks,
-- 
Tomoki Sekiyama <tomoki.sekiyama.qu@hitachi.com>
Linux Technology Center
Hitachi, Ltd., Yokohama Research Laboratory