[CHECKER] Does sys_sync (ext2, 2.6.x) flush metadata?

[CHECKER] Does sys_sync (ext2, 2.6.x) flush metadata?

[Junfeng Yang]

Hi,

We're working on a file system checker and have a question regarding what
sys_sync actually does.  It appears to us that sys_sync should sync both
data and metadata, and wait until both data and metadata hit the disk
before it returns.  Is this true for all the file systems (especially
ext2) for kernel 2.6.x?  I've gotten many "error" traces for ext2, where
directory entries are not flushed to disk after sys_sync.  In other words,
even if users do call sys_sync, a crash after sys_sync call can still
cause file losses.  Is this intended?

Thanks,
-Junfeng

Re: [CHECKER] Does sys_sync (ext2, 2.6.x) flush metadata?

[Andrew Morton]

Junfeng Yang <yjf@stanford.edu> wrote:
>
> 
> Hi,
> 
> We're working on a file system checker and have a question regarding what
> sys_sync actually does.  It appears to us that sys_sync should sync both
> data and metadata, and wait until both data and metadata hit the disk
> before it returns.  Is this true for all the file systems (especially
> ext2) for kernel 2.6.x?

It should be.

>  I've gotten many "error" traces for ext2, where
> directory entries are not flushed to disk after sys_sync.

How?

>  In other words,
> even if users do call sys_sync, a crash after sys_sync call can still
> cause file losses.  Is this intended?

No.

Re: [CHECKER] Does sys_sync (ext2, 2.6.x) flush metadata?

[Andi Kleen]

Junfeng Yang <yjf@stanford.edu> writes:

> We're working on a file system checker and have a question regarding what
> sys_sync actually does.  It appears to us that sys_sync should sync both
> data and metadata, and wait until both data and metadata hit the disk
> before it returns.  Is this true for all the file systems (especially
> ext2) for kernel 2.6.x?  I've gotten many "error" traces for ext2, where
> directory entries are not flushed to disk after sys_sync.  In other words,
> even if users do call sys_sync, a crash after sys_sync call can still
> cause file losses.  Is this intended?

No, it would be a bug. sync() is supposed to flush everything.
In the ext2 case directory changes should be hold in a dirty buffer,
which sync should normally flush.

-Andi

Re: [CHECKER] Does sys_sync (ext2, 2.6.x) flush metadata?

[Bartlomiej Zolnierkiewicz]

Hi,

On Thu, 10 Feb 2005 23:59:53 -0800 (PST), Junfeng Yang <yjf@stanford.edu> wrote:
> 
> Hi,
> 
> We're working on a file system checker and have a question regarding what
> sys_sync actually does.  It appears to us that sys_sync should sync both
> data and metadata, and wait until both data and metadata hit the disk
> before it returns.  Is this true for all the file systems (especially
> ext2) for kernel 2.6.x?  I've gotten many "error" traces for ext2, where
> directory entries are not flushed to disk after sys_sync.  In other words,
> even if users do call sys_sync, a crash after sys_sync call can still
> cause file losses.  Is this intended?

I don't know what exactly you are doing but do you remember
about disabling write caching on your disks in case of doing
real hard crashes?

Regards,
Bartlomiej