Re: [PATCH v9 30/34] qcow2: Add prealloc field to QCowL2Meta
On 03.07.20 01:05, Alberto Garcia wrote: > On Thu 02 Jul 2020 05:09:47 PM CEST, Max Reitz wrote: >>> Without a backing file, there is no read required - writing to an >>> unallocated subcluster within a preallocated cluster merely has to >>> provide zeros to the rest of the write. And depending on whether we >>> can intelligently guarantee that the underlying protocol already >>> reads as zeroes when preallocated, we even have an optimization where >>> even that is not necessary. We can still lump it in the "COW" >>> terminology, in that our write is more complex than merely writing in >>> place, but it isn't a true copy-on-write operation as there is >>> nothing to be copied. >> >> The term “COW” specifically in the qcow2 driver also refers to having >> to write zeroes to an area that isn’t written to by the guest as part >> of the process of having to allocate a (sub)cluster. > > The question is valid: if the space for the clusters is allocated but > the subclusters are not marked as such then any partial write request > will need to fill the rest with zeroes (in practice handle_alloc_space() > can do that efficiently but that's another question). > > If there is a backing file then there's no other alternative because we > do need to copy the data from the backing file. > > If there is no backing file perhaps we could allocate all subclusters as > well. I suppose we can detect that scenario at that point in the code (I > haven't checked) and I don't know what would happen if one later > attaches a backing file on runtime using the command-line options. > > But what I would argue is that I don't see the benefit of using extended > L2 entries on an preallocated image with no backing file: other than > having twice as much L2 metadata what would be the use? The point of > subclusters is that they make allocation more efficient, but if the > image is already fully allocated then they give you nothing. That’s true. I didn’t think about it this way. Then indeed it doesn’t make sense to potentially break cases of later adding a backing file: Reviewed-by: Max Reitz signature.asc Description: OpenPGP digital signature
Re: [PATCH v9 30/34] qcow2: Add prealloc field to QCowL2Meta
On Thu 02 Jul 2020 05:09:47 PM CEST, Max Reitz wrote: >> Without a backing file, there is no read required - writing to an >> unallocated subcluster within a preallocated cluster merely has to >> provide zeros to the rest of the write. And depending on whether we >> can intelligently guarantee that the underlying protocol already >> reads as zeroes when preallocated, we even have an optimization where >> even that is not necessary. We can still lump it in the "COW" >> terminology, in that our write is more complex than merely writing in >> place, but it isn't a true copy-on-write operation as there is >> nothing to be copied. > > The term “COW” specifically in the qcow2 driver also refers to having > to write zeroes to an area that isn’t written to by the guest as part > of the process of having to allocate a (sub)cluster. The question is valid: if the space for the clusters is allocated but the subclusters are not marked as such then any partial write request will need to fill the rest with zeroes (in practice handle_alloc_space() can do that efficiently but that's another question). If there is a backing file then there's no other alternative because we do need to copy the data from the backing file. If there is no backing file perhaps we could allocate all subclusters as well. I suppose we can detect that scenario at that point in the code (I haven't checked) and I don't know what would happen if one later attaches a backing file on runtime using the command-line options. But what I would argue is that I don't see the benefit of using extended L2 entries on an preallocated image with no backing file: other than having twice as much L2 metadata what would be the use? The point of subclusters is that they make allocation more efficient, but if the image is already fully allocated then they give you nothing. Berto
Re: [PATCH v9 30/34] qcow2: Add prealloc field to QCowL2Meta
On 02.07.20 16:58, Eric Blake wrote: > On 7/2/20 9:50 AM, Max Reitz wrote: >> On 28.06.20 13:02, Alberto Garcia wrote: >>> This field allows us to indicate that the L2 metadata update does not >>> come from a write request with actual data but from a preallocation >>> request. >>> >>> For traditional images this does not make any difference, but for >>> images with extended L2 entries this means that the clusters are >>> allocated normally in the L2 table but individual subclusters are >>> marked as unallocated. >>> >>> This will allow preallocating images that have a backing file. >>> >>> There is one special case: when we resize an existing image we can >>> also request that the new clusters are preallocated. If the image >>> already had a backing file then we have to hide any possible stale >>> data and zero out the new clusters (see commit 955c7d6687 for more >>> details). >>> >>> In this case the subclusters cannot be left as unallocated so the L2 >>> bitmap must be updated. >>> >>> Signed-off-by: Alberto Garcia >>> Reviewed-by: Eric Blake >>> --- >>> block/qcow2.h | 8 >>> block/qcow2-cluster.c | 2 +- >>> block/qcow2.c | 6 ++ >>> 3 files changed, 15 insertions(+), 1 deletion(-) >> >> Sounds good, but I’m just not quite sure about the details on >> falloc/full allocation: With .prealloc = true, writing to the >> preallocated subclusters will require a COW operation. That’s not >> ideal, and avoiding those COWs may be a reason to do preallocation in >> the first place. > > I'm not sure I follow the complaint. If a cluster is preallocated but > the subcluster is marked unallocated, then doing a partial write to that > subcluster must provide the correct contents for the rest of the > subcluster (either filling with zero, or reading from a backing file) - > but this COW can be limited to just the portion of the subcluster, and > is no different than the COW you have to perform without subclusters > when doing a write to a preallocated cluster in general. It was my impression that falloc/full preallocation would create normal data clusters, not zero clusters, so no COW was necessary when writing to them. >> Now, with backing files, it’s entirely correct. You need a COW >> operation, because that’s the point of having a backing file. >> >> But without a backing file I wonder if it wouldn’t be better to set >> .prealloc = false to avoid that COW. > > Without a backing file, there is no read required - writing to an > unallocated subcluster within a preallocated cluster merely has to > provide zeros to the rest of the write. And depending on whether we can > intelligently guarantee that the underlying protocol already reads as > zeroes when preallocated, we even have an optimization where even that > is not necessary. We can still lump it in the "COW" terminology, in > that our write is more complex than merely writing in place, but it > isn't a true copy-on-write operation as there is nothing to be copied. The term “COW” specifically in the qcow2 driver also refers to having to write zeroes to an area that isn’t written to by the guest as part of the process of having to allocate a (sub)cluster. (Of course there is no COW from a backing file if there is no backing file.) Max signature.asc Description: OpenPGP digital signature
Re: [PATCH v9 30/34] qcow2: Add prealloc field to QCowL2Meta
On 7/2/20 9:50 AM, Max Reitz wrote: On 28.06.20 13:02, Alberto Garcia wrote: This field allows us to indicate that the L2 metadata update does not come from a write request with actual data but from a preallocation request. For traditional images this does not make any difference, but for images with extended L2 entries this means that the clusters are allocated normally in the L2 table but individual subclusters are marked as unallocated. This will allow preallocating images that have a backing file. There is one special case: when we resize an existing image we can also request that the new clusters are preallocated. If the image already had a backing file then we have to hide any possible stale data and zero out the new clusters (see commit 955c7d6687 for more details). In this case the subclusters cannot be left as unallocated so the L2 bitmap must be updated. Signed-off-by: Alberto Garcia Reviewed-by: Eric Blake --- block/qcow2.h | 8 block/qcow2-cluster.c | 2 +- block/qcow2.c | 6 ++ 3 files changed, 15 insertions(+), 1 deletion(-) Sounds good, but I’m just not quite sure about the details on falloc/full allocation: With .prealloc = true, writing to the preallocated subclusters will require a COW operation. That’s not ideal, and avoiding those COWs may be a reason to do preallocation in the first place. I'm not sure I follow the complaint. If a cluster is preallocated but the subcluster is marked unallocated, then doing a partial write to that subcluster must provide the correct contents for the rest of the subcluster (either filling with zero, or reading from a backing file) - but this COW can be limited to just the portion of the subcluster, and is no different than the COW you have to perform without subclusters when doing a write to a preallocated cluster in general. Now, with backing files, it’s entirely correct. You need a COW operation, because that’s the point of having a backing file. But without a backing file I wonder if it wouldn’t be better to set .prealloc = false to avoid that COW. Without a backing file, there is no read required - writing to an unallocated subcluster within a preallocated cluster merely has to provide zeros to the rest of the write. And depending on whether we can intelligently guarantee that the underlying protocol already reads as zeroes when preallocated, we even have an optimization where even that is not necessary. We can still lump it in the "COW" terminology, in that our write is more complex than merely writing in place, but it isn't a true copy-on-write operation as there is nothing to be copied. Of course, if we did that, you couldn’t create the overlay separately from the backing file, preallocate it, and only then attach the backing file to the overlay. But is that a problem? (Or are there other problems to consider?) Max -- Eric Blake, Principal Software Engineer Red Hat, Inc. +1-919-301-3226 Virtualization: qemu.org | libvirt.org
Re: [PATCH v9 30/34] qcow2: Add prealloc field to QCowL2Meta
On 28.06.20 13:02, Alberto Garcia wrote: > This field allows us to indicate that the L2 metadata update does not > come from a write request with actual data but from a preallocation > request. > > For traditional images this does not make any difference, but for > images with extended L2 entries this means that the clusters are > allocated normally in the L2 table but individual subclusters are > marked as unallocated. > > This will allow preallocating images that have a backing file. > > There is one special case: when we resize an existing image we can > also request that the new clusters are preallocated. If the image > already had a backing file then we have to hide any possible stale > data and zero out the new clusters (see commit 955c7d6687 for more > details). > > In this case the subclusters cannot be left as unallocated so the L2 > bitmap must be updated. > > Signed-off-by: Alberto Garcia > Reviewed-by: Eric Blake > --- > block/qcow2.h | 8 > block/qcow2-cluster.c | 2 +- > block/qcow2.c | 6 ++ > 3 files changed, 15 insertions(+), 1 deletion(-) Sounds good, but I’m just not quite sure about the details on falloc/full allocation: With .prealloc = true, writing to the preallocated subclusters will require a COW operation. That’s not ideal, and avoiding those COWs may be a reason to do preallocation in the first place. Now, with backing files, it’s entirely correct. You need a COW operation, because that’s the point of having a backing file. But without a backing file I wonder if it wouldn’t be better to set .prealloc = false to avoid that COW. Of course, if we did that, you couldn’t create the overlay separately from the backing file, preallocate it, and only then attach the backing file to the overlay. But is that a problem? (Or are there other problems to consider?) Max signature.asc Description: OpenPGP digital signature
