Re: [PATCH 3/5] zfs com.delphix:hole_birth feature support

[Toomas Soome]

> On 19.04.2015, at 17:51, Andrei Borzenkov <address@hidden> wrote:
> 
> В Sun, 19 Apr 2015 17:40:16 +0300
> Toomas Soome <address@hidden> пишет:
> 
>> 
>> the features in openzfs have different effects, some affect only writes and 
>> are therefore read only compatible - such feature does not need any changes 
>> for reads, this feature is not read only compatible and therefore, once this 
>> feature is enabled, reader code must be changed accordingly.
>> 
>> so, what they did with hole_birth was they started to insert block birth 
>> timestamps for blocks being released to make it possible to track such 
>> blocks while doing zfs send (when block has birth time, you know its place 
>> on timeline of snapshots). as old reader code was relying on blk_birth == 0 
>> to detect an hole, after hole_birth is enabled, hole blk_birth is not 0 any 
>> more, so the solution is to check if DVA pointers are zero instead. and 
>> thats exactly what its about.  
> 
> And if hole_birth is *not* enabled on a filesystem we are reading?
> Should not old code be used in this case?


not really, as for holes, DVA pointers are  zero anyhow. I guess the original 
implementation used blk_birth to save extra compare - so the new read code is 
compatible with old data.


> 
>> 
>> reference:
>> https://github.com/illumos/illumos-gate/commit/43466aae47bfcd2ad9bf501faec8e75c08095e4f
>>  
>> 
>> 
>> 
>>> On 19.04.2015, at 17:11, Andrei Borzenkov <address@hidden> wrote:
>>> 
>>> В Thu, 16 Apr 2015 08:22:08 +0300
>>> Toomas Soome <address@hidden> пишет:
>>> 
>>> This really needs better explanation. Otherwise this looks like either
>>> old code was broken to start with and it is a bug fix or new code needs
>>> some conditionals on new feature.
>>> 
>>>> 
>>>> ---
>>>> grub-core/fs/zfs/zfs.c |    6 ++++--
>>>> include/grub/zfs/spa.h |    4 +++-
>>>> 2 files changed, 7 insertions(+), 3 deletions(-)
>>>> 
>>>> diff --git a/grub-core/fs/zfs/zfs.c b/grub-core/fs/zfs/zfs.c
>>>> index 2689986..a731c3d 100644
>>>> --- a/grub-core/fs/zfs/zfs.c
>>>> +++ b/grub-core/fs/zfs/zfs.c
>>>> @@ -280,7 +280,9 @@ grub_crypto_cipher_handle_t (*grub_zfs_load_key) 
>>>> (const struct grub_zfs_key *key
>>>> */
>>>> #define MAX_SUPPORTED_FEATURE_STRLEN 50
>>>> static const char *spa_feature_names[] = {
>>>> -  "org.illumos:lz4_compress",NULL
>>>> +  "org.illumos:lz4_compress",
>>>> +  "com.delphix:hole_birth",
>>>> +  NULL
>>>> };
>>>> 
>>>> static int
>>>> @@ -1751,7 +1753,7 @@ zio_read_gang (blkptr_t * bp, grub_zfs_endian_t 
>>>> endian, dva_t * dva, void *buf,
>>>> 
>>>>  for (i = 0; i < SPA_GBH_NBLKPTRS; i++)
>>>>    {
>>>> -      if (zio_gb->zg_blkptr[i].blk_birth == 0)
>>>> +      if (BP_IS_HOLE(&zio_gb->zg_blkptr[i]))
>>>>    continue;
>>>> 
>>>>      err = zio_read_data (&zio_gb->zg_blkptr[i], endian, buf, data);
>>>> diff --git a/include/grub/zfs/spa.h b/include/grub/zfs/spa.h
>>>> index 7edb8ab..df43b6b 100644
>>>> --- a/include/grub/zfs/spa.h
>>>> +++ b/include/grub/zfs/spa.h
>>>> @@ -279,7 +279,9 @@ typedef struct blkptr {
>>>> 
>>>> #define    BP_IDENTITY(bp)         (&(bp)->blk_dva[0])
>>>> #define    BP_IS_GANG(bp)          DVA_GET_GANG(BP_IDENTITY(bp))
>>>> -#define   BP_IS_HOLE(bp)          ((bp)->blk_birth == 0)
>>>> +#define   DVA_IS_EMPTY(dva)       ((dva)->dva_word[0] == 0ULL && \
>>>> +                          (dva)->dva_word[1] == 0ULL)
>>>> +#define   BP_IS_HOLE(bp)          DVA_IS_EMPTY(BP_IDENTITY(bp))
>>>> 
>>>> /* BP_IS_RAIDZ(bp) assumes no block compression */
>>>> #define    BP_IS_RAIDZ(bp)         (DVA_GET_ASIZE(&(bp)->blk_dva[0]) > \
>>> 
>>> 
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>> 
>> 
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