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


From: Toomas Soome
Subject: Re: [PATCH 3/5] zfs com.delphix:hole_birth feature support
Date: Sun, 19 Apr 2015 17:40:16 +0300

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.  

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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