maria-discuss team mailing list archive

[Justin Swanhart <greenlion@xxxxxxxxx>]

Hi,

InnoDB recovery can not handle torn pages.  An fsync is required to ensure
that the page is fully written to disk.  This is also why the doublewrite
buffer is used.  Before pages are written down to disk, they are first
written sequentially into the doublewrite buffer.  This buffer is synced,
then async page writing can proceed.  If the database crashes, the pages in
flight will be rewritten by the doublewrite buffer.  The detection
mechanism for torn pages comes from an LSN, which is written into the top
and the bottom of the page.  If the LSN at the top and bottom do not match
the page is torn.

Regards,

--Justin

On Fri, May 8, 2015 at 12:43 PM, Xiaofei Du <xiaofei.du008@xxxxxxxxx> wrote:

> Laurynas,
>
> This is exactly what I was looking for. I went through these functions
> before. I disabled double write buffer, so I didn't pay attention to code
> under buf_dblwr... The reason I asked this question is because I didn't
> know how the recovery process works, so I was wondering if it's necessary
> to fsync after each write. It's a performance concern. Anyway, thank you
> very much!
>
> Jan -- Thank you for your answer too!
>
> Xiaofei
>
> On Thu, May 7, 2015 at 9:59 PM, Laurynas Biveinis <
> laurynas.biveinis@xxxxxxxxx> wrote:
>
>> Xiaofei -
>>
>> fsync is performed for all the flush types (LRU, flush, single page)
>> if it is asked for (innodb_flush_method != O_DIRECT_NO_FSYNC). The
>> apparent difference in sync and async is not because of the sync
>> difference itself, but because of the flush type difference. The
>> single page flush flushes one page, and requests a fsync for its file.
>> Other flushes flush in batches, don't have to fsync for each written
>> page individually but rather sync once at the end. Then doublewrite
>> complicates this further. If it is disabled, fsync will happen in
>> buf_dblwr_sync_datafiles called from buf_dblwr_flush_buffered_writes
>> called from buf_flush_common called at the end of either LRU or flush
>> list flush. If doublewrite is enabled, fsync will happen in
>> buf_dblwr_update called from buf_flush_write_complete.
>>
>>
>>
>>
>> 2015-05-07 9:01 GMT+03:00 Xiaofei Du <xiaofei.du008@xxxxxxxxx>:
>> > Hi Laurynas,
>> >
>> > On Wed, May 6, 2015 at 9:14 PM, Laurynas Biveinis
>> > <laurynas.biveinis@xxxxxxxxx> wrote:
>> >>
>> >> Xiaofei -
>> >>
>> >> > Does InnoDB maintain a dirty
>> >> > page table?
>> >>
>> >> You must be referring to the buffer pool flush_list.
>> >
>> >
>> > You are right. The flush_list is can be used for recovery and
>> checkpoint.
>> >
>> >>
>> >>
>> >> > Is fsync called to guarantee the page to be on persistent
>> >> > storage so that the dirty page table can be updated? If this is the
>> >> > case,
>> >> > when is the dirty page table updated for asynchronous IOs?
>> >>
>> >> Check buf_flush_write_complete in buf0flu.cc. For async IO it is
>> >> called from buf_page_io_complete in buf0buf.cc.
>> >
>> >
>> > You are right that this is the place it updates the dirty page
>> information.
>> > But I still don't understand why the fsync is needed for synchronous
>> IOs,
>> > but not for the AIOs. Jan Lindstrom said fsync is also called for other
>> AIO
>> > operations. But I could only it true in one of many AIO operations. Or
>> maybe
>> > I am missing something still?
>> >
>> >>
>> >>
>> >> --
>> >> Laurynas
>> >
>> >
>>
>>
>>
>> --
>> Laurynas
>>
>
>
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