Help two questions about update_global_block_rsv()
update_global_block_rsv(fs_info) {
...
block_rsv = &fs_info->global_block_rsv;
sinfo = block_rsv->space_info;
num_bytes = sinfo->bytes_used + sinfo->bytes_pinned +
sinfo->bytes_reserved + sinfo->bytes_readonly +
sinfo->bytes_may_use;
if (sinfo->total_bytes > num_bytes) {
num_bytes = sinfo->total_bytes - num_bytes;
block_rsv->reserved += num_bytes;
sinfo->bytes_may_use += num_bytes;
}
...
}
1. I don''t quite understand the lines:
block_rsv->reserved += num_bytes;
sinfo->bytes_may_use += num_bytes;
why aren''t they like these?
block_rsv->reserved = num_bytes;
sinfo->bytes_may_use = num_bytes;
assuming that nothing else is changed, and we just call
update_global_block_rsv() for several times, then block_rsv->reserved will
increase constantly, and so sinfo->bytes_may_use; can anyone tell me what it
means? I mean if nothing else is changed the two variables should keep their
values unchanged also, is that right? a bit puzzlement to me.
2. Is the meaning of sinfo->bytes_may_use that bytes which is available for
use
or that bytes which will likely/unlikely be used?
thanks
WeiFeng Liu
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On 06/07/2012 10:46 PM, WeiFeng Liu wrote:> Help two questions about update_global_block_rsv() > > update_global_block_rsv(fs_info) { > > ... > block_rsv = &fs_info->global_block_rsv; > sinfo = block_rsv->space_info; > > num_bytes = sinfo->bytes_used + sinfo->bytes_pinned + > sinfo->bytes_reserved + sinfo->bytes_readonly + > sinfo->bytes_may_use; > > if (sinfo->total_bytes > num_bytes) { > num_bytes = sinfo->total_bytes - num_bytes; > block_rsv->reserved += num_bytes; > sinfo->bytes_may_use += num_bytes; > } > ... > } > > 1. I don''t quite understand the lines: > block_rsv->reserved += num_bytes; > sinfo->bytes_may_use += num_bytes; > > why aren''t they like these? > block_rsv->reserved = num_bytes; > sinfo->bytes_may_use = num_bytes; >> assuming that nothing else is changed, and we just call > update_global_block_rsv() for several times, then block_rsv->reserved will > increase constantly, and so sinfo->bytes_may_use; can anyone tell me what it > means? I mean if nothing else is changed the two variables should keep their > values unchanged also, is that right? a bit puzzlement to me. >the following if-statement tells the truth: if (block_rsv->reserved >= block_rsv->size) { ... }> 2. Is the meaning of sinfo->bytes_may_use that bytes which is available for use > or that bytes which will likely/unlikely be used? >You need to understand how a block_rsv works, both block_rsv and space_info are a kind of _container_: Introducing metadata reseravtion contexts has two major advantages. First, it makes metadata reservation more traceable. Second, it can reclaim freed space and re-add them to the itself after transaction committed. here is a basic model: +--+--->size +--+--->size +--------------+ | | | | | | | | | | | | +--+--->reserved +--+--->reserved +--------------+-->may_used | | | | | | | | | | | | | | | | | | +--+ +--+ +--------------+ block_rsv + block_rsv + ... = space_info(flag=4) A B | | +--> reserved meta bytes Hope that this helps. thanks, liubo> thanks > > WeiFeng Liu > > -- > To unsubscribe from this list: send the line "unsubscribe linux-btrfs" in > the body of a message to majordomo@vger.kernel.org > More majordomo info at http://vger.kernel.org/majordomo-info.html >-- To unsubscribe from this list: send the line "unsubscribe linux-btrfs" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html
> >You need to understand how a block_rsv works, > >both block_rsv and space_info are a kind of _container_: > > Introducing metadata reseravtion contexts has two major >advantages. > First, it makes metadata reservation more traceable. > Second, it can reclaim freed space and re-add them to the >itself after transaction committed. > > >here is a basic model: > > +--+--->size +--+--->size +--- >-----------+ > | | | | | > | > | | | | | > | > +--+--->reserved +--+--->reserved +--- >-----------+-->may_used > | | | | | > | > | | | | | > | > | | | | | > | > +--+ +--+ +--- >-----------+ > block_rsv + block_rsv + ... = >space_info(flag=4) > A B | > | > +--> reserved >meta bytes > > >Hope that this helps. > >thanks, >liuboI''m afraid it''s a bit late to say thanks, LiuBo. I saw your June 11 replay yesterday, I have not touched my mail account for nearly ten days since my original post, I thought it would have been burried by time after I kept watching someone''s replay for 2, 3 days and failed to get one. your illustration is usefull for me, I will re-browse some rsv code with it. thank you very much, WeiFeng Liu -- To unsubscribe from this list: send the line "unsubscribe linux-btrfs" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html