linux-kernel - Re: [PATCH v2] ceph: allow object copies across different filesystems in the same cluster

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Date:   Mon, 09 Sep 2019 12:15:16 +0100
From:   Luis Henriques <lhenriques@...e.com>
To:     "Jeff Layton" <jlayton@...nel.org>
Cc:     "IlyaDryomov" <idryomov@...il.com>, "Sage Weil" <sage@...hat.com>,
        <ceph-devel@...r.kernel.org>, <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v2] ceph: allow object copies across different filesystems in the same cluster

"Jeff Layton" <jlayton@...nel.org> writes:

> On Mon, 2019-09-09 at 11:28 +0100, Luis Henriques wrote:
>> OSDs are able to perform object copies across different pools.  Thus,
>> there's no need to prevent copy_file_range from doing remote copies if the
>> source and destination superblocks are different.  Only return -EXDEV if
>> they have different fsid (the cluster ID).
>> 
>> Signed-off-by: Luis Henriques <lhenriques@...e.com>
>> ---
>>  fs/ceph/file.c | 18 ++++++++++++++----
>>  1 file changed, 14 insertions(+), 4 deletions(-)
>> 
>> Hi,
>> 
>> Here's the patch changelog since initial submittion:
>> 
>> - Dropped have_fsid checks on client structs
>> - Use %pU to print the fsid instead of raw hex strings (%*ph)
>> - Fixed 'To:' field in email so that this time the patch hits vger
>> 
>> Cheers,
>> --
>> Luis
>> 
>> diff --git a/fs/ceph/file.c b/fs/ceph/file.c
>> index 685a03cc4b77..4a624a1dd0bb 100644
>> --- a/fs/ceph/file.c
>> +++ b/fs/ceph/file.c
>> @@ -1904,6 +1904,7 @@ static ssize_t __ceph_copy_file_range(struct file *src_file, loff_t src_off,
>>  	struct ceph_inode_info *src_ci = ceph_inode(src_inode);
>>  	struct ceph_inode_info *dst_ci = ceph_inode(dst_inode);
>>  	struct ceph_cap_flush *prealloc_cf;
>> +	struct ceph_fs_client *src_fsc = ceph_inode_to_client(src_inode);
>>  	struct ceph_object_locator src_oloc, dst_oloc;
>>  	struct ceph_object_id src_oid, dst_oid;
>>  	loff_t endoff = 0, size;
>> @@ -1915,8 +1916,17 @@ static ssize_t __ceph_copy_file_range(struct file *src_file, loff_t src_off,
>>  
>>  	if (src_inode == dst_inode)
>>  		return -EINVAL;
>> -	if (src_inode->i_sb != dst_inode->i_sb)
>> -		return -EXDEV;
>> +	if (src_inode->i_sb != dst_inode->i_sb) {
>> +		struct ceph_fs_client *dst_fsc = ceph_inode_to_client(dst_inode);
>> +
>> +		if (ceph_fsid_compare(&src_fsc->client->fsid,
>> +				      &dst_fsc->client->fsid)) {
>> +			dout("Copying object across different clusters:");
>> +			dout("  src fsid: %pU dst fsid: %pU\n",
>> +			     &src_fsc->client->fsid, &dst_fsc->client->fsid);
>> +			return -EXDEV;
>> +		}
>> +	}
>
> Just to be clear: what happens here if I mount two entirely separate
> clusters, and their OSDs don't have any access to one another? Will this
> fail at some later point with an error that we can catch so that we can
> fall back?

This is exactly what this check prevents: if we have two CephFS from two
unrelated clusters mounted and we try to copy a file across them, the
operation will fail with -EXDEV[1] because the FSIDs for these two
ceph_fs_client will be different.  OTOH, if these two filesystems are
within the same cluster (and thus with the same FSID), then the OSDs are
able to do 'copy-from' operations between them.

I've tested all these scenarios and they seem to be handled correctly.
Now, I'm assuming that *all* OSDs within the same ceph cluster can
communicate between themselves; if this assumption is false, then this
patch is broken.  But again, I'm not aware of any mechanism that
prevents 2 OSDs from communicating between them.

[1] Actually, the files will still be copied because we'll fallback into
the default VFS generic_copy_file_range behaviour, which is to do
reads+writes operations.

Cheers,
-- 
Luis


>
>
>>  	if (ceph_snap(dst_inode) != CEPH_NOSNAP)
>>  		return -EROFS;
>>  
>> @@ -1928,7 +1938,7 @@ static ssize_t __ceph_copy_file_range(struct file *src_file, loff_t src_off,
>>  	 * efficient).
>>  	 */
>>  
>> -	if (ceph_test_mount_opt(ceph_inode_to_client(src_inode), NOCOPYFROM))
>> +	if (ceph_test_mount_opt(src_fsc, NOCOPYFROM))
>>  		return -EOPNOTSUPP;
>>  
>>  	if ((src_ci->i_layout.stripe_unit != dst_ci->i_layout.stripe_unit) ||
>> @@ -2044,7 +2054,7 @@ static ssize_t __ceph_copy_file_range(struct file *src_file, loff_t src_off,
>>  				dst_ci->i_vino.ino, dst_objnum);
>>  		/* Do an object remote copy */
>>  		err = ceph_osdc_copy_from(
>> -			&ceph_inode_to_client(src_inode)->client->osdc,
>> +			&src_fsc->client->osdc,
>>  			src_ci->i_vino.snap, 0,
>>  			&src_oid, &src_oloc,
>>  			CEPH_OSD_OP_FLAG_FADVISE_SEQUENTIAL |