Re: [Libguestfs] [RFC] lib: allow to specify physical/logical block size for disks

Re: [Libguestfs] [RFC] lib: allow to specify physical/logical block size for disks

[Richard W.M. Jones]

On Sat, Feb 08, 2020 at 01:25:28AM +0200, Mykola Ivanets wrote:
> From: Nikolay Ivanets <stenavin@gmail.com>
> 
> I faced with situation where libguestfs cannot recognize partitions on a
> disk image which was partitioned on a system with "4K native" sector
> size support.

Do you have a small test case for this?

> In order to fix the issue we need to allow users to specify desired
> physical and/or logical block size per drive basis.

It seems like physical_block_size / logical_block_size in qemu are
completely undocumented.  However I did some experiments with patching
libguestfs and examining the qemu and parted code.  Here are my
observations:

(1) Setting only physical_block_size = 4096 seems to do nothing.

(2) Setting only logical_block_size = 4096 is explicitly rejected by
virtio-scsi:

https://git.qemu.org/?p=qemu.git;a=blob;f=hw/scsi/scsi-disk.c;h=10d0794d60f196f177563aae00bed2181f5c1bb1;hb=HEAD#l2352

(A similar test exists for virtio-blk)

(3) Setting both physical_block_size = logical_block_size = 4096
changes how parted partitions GPT disks.  The partition table is
clearly using 4K sectors as you can see by examining the disk
afterwards with hexdump.

(4) Neither setting changes MBR partitioning by parted, although my
interpretation of Wikipedia indicates that it should be possible to
create a MBR disk with 4K sector size.  Maybe I'm doing something
wrong, or parted just doesn't support this case.

So it appears that we should just have one blocksize control (maybe
called "sectorsize"?) which sets both physical_block_size and
logical_block_size to the same value.  It may also be worth enforcing
that blocksize/sectorsize must be set to 512 or 4096 (which we can
relax later if necessary).

Rich.

-- 
Richard Jones, Virtualization Group, Red Hat http://people.redhat.com/~rjones
Read my programming and virtualization blog: http://rwmj.wordpress.com
virt-p2v converts physical machines to virtual machines.  Boot with a
live CD or over the network (PXE) and turn machines into KVM guests.
http://libguestfs.org/virt-v2v

Re: [Libguestfs] [RFC] lib: allow to specify physical/logical block size for disks

[Nikolay Ivanets]

пн, 10 лют. 2020 о 13:43 Richard W.M. Jones <rjones@redhat.com> пише:
>
> On Sat, Feb 08, 2020 at 01:25:28AM +0200, Mykola Ivanets wrote:
> > From: Nikolay Ivanets <stenavin@gmail.com>
> >
> > I faced with situation where libguestfs cannot recognize partitions on a
> > disk image which was partitioned on a system with "4K native" sector
> > size support.
>
> Do you have a small test case for this?

We can easily create one with patched libguestfs and attach disk to
unpatched libguestfs.

> > In order to fix the issue we need to allow users to specify desired
> > physical and/or logical block size per drive basis.
>
> It seems like physical_block_size / logical_block_size in qemu are
> completely undocumented.  However I did some experiments with patching
> libguestfs and examining the qemu and parted code.  Here are my
> observations:
>
> (1) Setting only physical_block_size = 4096 seems to do nothing.

See my thoughts on this in previous email.

> (2) Setting only logical_block_size = 4096 is explicitly rejected by
> virtio-scsi:
>
> https://git.qemu.org/?p=qemu.git;a=blob;f=hw/scsi/scsi-disk.c;h=10d0794d60f196f177563aae00bed2181f5c1bb1;hb=HEAD#l2352
>
> (A similar test exists for virtio-blk)
>
> (3) Setting both physical_block_size = logical_block_size = 4096
> changes how parted partitions GPT disks.  The partition table is
> clearly using 4K sectors as you can see by examining the disk
> afterwards with hexdump.
>
> (4) Neither setting changes MBR partitioning by parted, although my
> interpretation of Wikipedia indicates that it should be possible to
> create a MBR disk with 4K sector size.  Maybe I'm doing something
> wrong, or parted just doesn't support this case.
>
> So it appears that we should just have one blocksize control (maybe
> called "sectorsize"?) which sets both physical_block_size and
> logical_block_size to the same value.  It may also be worth enforcing
> that blocksize/sectorsize must be set to 512 or 4096 (which we can
> relax later if necessary).

If we stick with the only parameter, I think blocksize might be better name,
especially if we want to split this parameter somewhere latter.

Here are more precise restrictions:

Both values must be a power of 2 between 512 and 32768.
logical_block_size must be
less or equals to physical_block_size.

--
  Mykola Ivanets

Re: [Libguestfs] [RFC] lib: allow to specify physical/logical block size for disks

[Richard W.M. Jones]

On Mon, Feb 10, 2020 at 02:28:08PM +0200, Nikolay Ivanets wrote:
> пн, 10 лют. 2020 о 13:43 Richard W.M. Jones <rjones@redhat.com> пише:
> >
> > On Sat, Feb 08, 2020 at 01:25:28AM +0200, Mykola Ivanets wrote:
> > > From: Nikolay Ivanets <stenavin@gmail.com>
> > >
> > > I faced with situation where libguestfs cannot recognize partitions on a
> > > disk image which was partitioned on a system with "4K native" sector
> > > size support.
> >
> > Do you have a small test case for this?
> 
> We can easily create one with patched libguestfs and attach disk to
> unpatched libguestfs.
> 
> > > In order to fix the issue we need to allow users to specify desired
> > > physical and/or logical block size per drive basis.
> >
> > It seems like physical_block_size / logical_block_size in qemu are
> > completely undocumented.  However I did some experiments with patching
> > libguestfs and examining the qemu and parted code.  Here are my
> > observations:
> >
> > (1) Setting only physical_block_size = 4096 seems to do nothing.
> 
> See my thoughts on this in previous email.
> 
> > (2) Setting only logical_block_size = 4096 is explicitly rejected by
> > virtio-scsi:
> >
> > https://git.qemu.org/?p=qemu.git;a=blob;f=hw/scsi/scsi-disk.c;h=10d0794d60f196f177563aae00bed2181f5c1bb1;hb=HEAD#l2352
> >
> > (A similar test exists for virtio-blk)
> >
> > (3) Setting both physical_block_size = logical_block_size = 4096
> > changes how parted partitions GPT disks.  The partition table is
> > clearly using 4K sectors as you can see by examining the disk
> > afterwards with hexdump.
> >
> > (4) Neither setting changes MBR partitioning by parted, although my
> > interpretation of Wikipedia indicates that it should be possible to
> > create a MBR disk with 4K sector size.  Maybe I'm doing something
> > wrong, or parted just doesn't support this case.
> >
> > So it appears that we should just have one blocksize control (maybe
> > called "sectorsize"?) which sets both physical_block_size and
> > logical_block_size to the same value.  It may also be worth enforcing
> > that blocksize/sectorsize must be set to 512 or 4096 (which we can
> > relax later if necessary).
> 
> If we stick with the only parameter, I think blocksize might be better name,
> especially if we want to split this parameter somewhere latter.
> 
> Here are more precise restrictions:
> 
> Both values must be a power of 2 between 512 and 32768.
> logical_block_size must be
> less or equals to physical_block_size.

Agreed, but note that we can relax restrictions later if we want, but
enforcing restrictions later is an ABI break.

The only disk format I'm aware of which uses !512 and !4K sectors are
CD ROMs (2K sector size), although libguestfs reads those without any
problems today.  Even if you consider NASes where 64K sectors are
normal, they still use 512 or 4K logical sectors (with lots of
horrible read-modify-write cycles).

Rich.

-- 
Richard Jones, Virtualization Group, Red Hat http://people.redhat.com/~rjones
Read my programming and virtualization blog: http://rwmj.wordpress.com
virt-p2v converts physical machines to virtual machines.  Boot with a
live CD or over the network (PXE) and turn machines into KVM guests.
http://libguestfs.org/virt-v2v

Re: [Libguestfs] [RFC] lib: allow to specify physical/logical block size for disks

[Kevin Wolf]

Am 10.02.2020 um 12:43 hat Richard W.M. Jones geschrieben:
> On Sat, Feb 08, 2020 at 01:25:28AM +0200, Mykola Ivanets wrote:
> > From: Nikolay Ivanets <stenavin@gmail.com>
> > 
> > I faced with situation where libguestfs cannot recognize partitions on a
> > disk image which was partitioned on a system with "4K native" sector
> > size support.
> 
> Do you have a small test case for this?
> 
> > In order to fix the issue we need to allow users to specify desired
> > physical and/or logical block size per drive basis.
> 
> It seems like physical_block_size / logical_block_size in qemu are
> completely undocumented.  However I did some experiments with patching
> libguestfs and examining the qemu and parted code.  Here are my
> observations:
> 
> (1) Setting only physical_block_size = 4096 seems to do nothing.

The guest sees the physical_block_size and can try to keep its requests
aligned as an optimisation. But it doesn't actually make a semantic
difference as to how the content of the disk is accessed.

> (2) Setting only logical_block_size = 4096 is explicitly rejected by
> virtio-scsi:
> 
> https://git.qemu.org/?p=qemu.git;a=blob;f=hw/scsi/scsi-disk.c;h=10d0794d60f196f177563aae00bed2181f5c1bb1;hb=HEAD#l2352
> 
> (A similar test exists for virtio-blk)
> 
> (3) Setting both physical_block_size = logical_block_size = 4096
> changes how parted partitions GPT disks.  The partition table is
> clearly using 4K sectors as you can see by examining the disk
> afterwards with hexdump.

This is what you want for emulating a 4k native disk.

> (4) Neither setting changes MBR partitioning by parted, although my
> interpretation of Wikipedia indicates that it should be possible to
> create a MBR disk with 4K sector size.  Maybe I'm doing something
> wrong, or parted just doesn't support this case.

I seem to remember that 4k native disks require GPT, but if you say you
read otherwise, I'm not 100% sure about this any more.

> So it appears that we should just have one blocksize control (maybe
> called "sectorsize"?) which sets both physical_block_size and
> logical_block_size to the same value.  It may also be worth enforcing
> that blocksize/sectorsize must be set to 512 or 4096 (which we can
> relax later if necessary).

A single option (to control logical_block_size) makes sense for
libguestfs. physical_block_size is only relevant for the appliance and
not for the resulting image, so it can be treated as an implementation
detail.

Kevin

Re: [Libguestfs] [RFC] lib: allow to specify physical/logical block size for disks

[Nikolay Ivanets]

пн, 10 лют. 2020 о 15:02 Richard W.M. Jones <rjones@redhat.com> пише:
>
> On Mon, Feb 10, 2020 at 02:28:08PM +0200, Nikolay Ivanets wrote:
> > пн, 10 лют. 2020 о 13:43 Richard W.M. Jones <rjones@redhat.com> пише:
> > >
> > > On Sat, Feb 08, 2020 at 01:25:28AM +0200, Mykola Ivanets wrote:
> > > > From: Nikolay Ivanets <stenavin@gmail.com>
> > > >
> > > > I faced with situation where libguestfs cannot recognize partitions on a
> > > > disk image which was partitioned on a system with "4K native" sector
> > > > size support.
> > >
> > > Do you have a small test case for this?
> >
> > We can easily create one with patched libguestfs and attach disk to
> > unpatched libguestfs.
> >
> > > > In order to fix the issue we need to allow users to specify desired
> > > > physical and/or logical block size per drive basis.
> > >
> > > It seems like physical_block_size / logical_block_size in qemu are
> > > completely undocumented.  However I did some experiments with patching
> > > libguestfs and examining the qemu and parted code.  Here are my
> > > observations:
> > >
> > > (1) Setting only physical_block_size = 4096 seems to do nothing.
> >
> > See my thoughts on this in previous email.
> >
> > > (2) Setting only logical_block_size = 4096 is explicitly rejected by
> > > virtio-scsi:
> > >
> > > https://git.qemu.org/?p=qemu.git;a=blob;f=hw/scsi/scsi-disk.c;h=10d0794d60f196f177563aae00bed2181f5c1bb1;hb=HEAD#l2352
> > >
> > > (A similar test exists for virtio-blk)
> > >
> > > (3) Setting both physical_block_size = logical_block_size = 4096
> > > changes how parted partitions GPT disks.  The partition table is
> > > clearly using 4K sectors as you can see by examining the disk
> > > afterwards with hexdump.
> > >
> > > (4) Neither setting changes MBR partitioning by parted, although my
> > > interpretation of Wikipedia indicates that it should be possible to
> > > create a MBR disk with 4K sector size.  Maybe I'm doing something
> > > wrong, or parted just doesn't support this case.
> > >
> > > So it appears that we should just have one blocksize control (maybe
> > > called "sectorsize"?) which sets both physical_block_size and
> > > logical_block_size to the same value.  It may also be worth enforcing
> > > that blocksize/sectorsize must be set to 512 or 4096 (which we can
> > > relax later if necessary).
> >
> > If we stick with the only parameter, I think blocksize might be better name,
> > especially if we want to split this parameter somewhere latter.
> >
> > Here are more precise restrictions:
> >
> > Both values must be a power of 2 between 512 and 32768.
> > logical_block_size must be
> > less or equals to physical_block_size.
>
> Agreed, but note that we can relax restrictions later if we want, but
> enforcing restrictions later is an ABI break.
>
> The only disk format I'm aware of which uses !512 and !4K sectors are
> CD ROMs (2K sector size), although libguestfs reads those without any
> problems today.  Even if you consider NASes where 64K sectors are
> normal, they still use 512 or 4K logical sectors (with lots of
> horrible read-modify-write cycles).

In this case we will reject libvirt XML with block size other then 512 and 4096.
I'm fine with that because other values are artificial.

Re: [Libguestfs] [RFC] lib: allow to specify physical/logical block size for disks

[Nikolay Ivanets]

пн, 10 лют. 2020 о 15:48 Kevin Wolf <kwolf@redhat.com> пише:
>
> Am 10.02.2020 um 12:43 hat Richard W.M. Jones geschrieben:
> > On Sat, Feb 08, 2020 at 01:25:28AM +0200, Mykola Ivanets wrote:
> > > From: Nikolay Ivanets <stenavin@gmail.com>
> > >
> > > I faced with situation where libguestfs cannot recognize partitions on a
> > > disk image which was partitioned on a system with "4K native" sector
> > > size support.
> >
> > Do you have a small test case for this?
> >
> > > In order to fix the issue we need to allow users to specify desired
> > > physical and/or logical block size per drive basis.
> >
> > It seems like physical_block_size / logical_block_size in qemu are
> > completely undocumented.  However I did some experiments with patching
> > libguestfs and examining the qemu and parted code.  Here are my
> > observations:
> >
> > (1) Setting only physical_block_size = 4096 seems to do nothing.
>
> The guest sees the physical_block_size and can try to keep its requests
> aligned as an optimisation. But it doesn't actually make a semantic
> difference as to how the content of the disk is accessed.
>
> > (2) Setting only logical_block_size = 4096 is explicitly rejected by
> > virtio-scsi:
> >
> > https://git.qemu.org/?p=qemu.git;a=blob;f=hw/scsi/scsi-disk.c;h=10d0794d60f196f177563aae00bed2181f5c1bb1;hb=HEAD#l2352
> >
> > (A similar test exists for virtio-blk)
> >
> > (3) Setting both physical_block_size = logical_block_size = 4096
> > changes how parted partitions GPT disks.  The partition table is
> > clearly using 4K sectors as you can see by examining the disk
> > afterwards with hexdump.
>
> This is what you want for emulating a 4k native disk.
>
> > (4) Neither setting changes MBR partitioning by parted, although my
> > interpretation of Wikipedia indicates that it should be possible to
> > create a MBR disk with 4K sector size.  Maybe I'm doing something
> > wrong, or parted just doesn't support this case.
>
> I seem to remember that 4k native disks require GPT, but if you say you
> read otherwise, I'm not 100% sure about this any more.
>
> > So it appears that we should just have one blocksize control (maybe
> > called "sectorsize"?) which sets both physical_block_size and
> > logical_block_size to the same value.  It may also be worth enforcing
> > that blocksize/sectorsize must be set to 512 or 4096 (which we can
> > relax later if necessary).
>
> A single option (to control logical_block_size) makes sense for
> libguestfs. physical_block_size is only relevant for the appliance and
> not for the resulting image, so it can be treated as an implementation
> detail.
>
> Kevin
>

So, can we summarize?

- in libguestfs we will expose the only parameter called 'blocksize'
- 512 and 4096 are the only allowed values for 'blocksize' for now
- we will reject libvirt XML with values for physical_* and
logical_block_size other then 512 or 4096
- importing disks configuration from libvirt XML we will use logical_block_size

Richard, are we fine with that?

--
  Mykola Ivanets

Re: [Libguestfs] [RFC] lib: allow to specify physical/logical block size for disks

[Richard W.M. Jones]

On Mon, Feb 10, 2020 at 04:15:40PM +0200, Nikolay Ivanets wrote:
> пн, 10 лют. 2020 о 15:48 Kevin Wolf <kwolf@redhat.com> пише:
> >
> > Am 10.02.2020 um 12:43 hat Richard W.M. Jones geschrieben:
> > > On Sat, Feb 08, 2020 at 01:25:28AM +0200, Mykola Ivanets wrote:
> > > > From: Nikolay Ivanets <stenavin@gmail.com>
> > > >
> > > > I faced with situation where libguestfs cannot recognize partitions on a
> > > > disk image which was partitioned on a system with "4K native" sector
> > > > size support.
> > >
> > > Do you have a small test case for this?
> > >
> > > > In order to fix the issue we need to allow users to specify desired
> > > > physical and/or logical block size per drive basis.
> > >
> > > It seems like physical_block_size / logical_block_size in qemu are
> > > completely undocumented.  However I did some experiments with patching
> > > libguestfs and examining the qemu and parted code.  Here are my
> > > observations:
> > >
> > > (1) Setting only physical_block_size = 4096 seems to do nothing.
> >
> > The guest sees the physical_block_size and can try to keep its requests
> > aligned as an optimisation. But it doesn't actually make a semantic
> > difference as to how the content of the disk is accessed.
> >
> > > (2) Setting only logical_block_size = 4096 is explicitly rejected by
> > > virtio-scsi:
> > >
> > > https://git.qemu.org/?p=qemu.git;a=blob;f=hw/scsi/scsi-disk.c;h=10d0794d60f196f177563aae00bed2181f5c1bb1;hb=HEAD#l2352
> > >
> > > (A similar test exists for virtio-blk)
> > >
> > > (3) Setting both physical_block_size = logical_block_size = 4096
> > > changes how parted partitions GPT disks.  The partition table is
> > > clearly using 4K sectors as you can see by examining the disk
> > > afterwards with hexdump.
> >
> > This is what you want for emulating a 4k native disk.
> >
> > > (4) Neither setting changes MBR partitioning by parted, although my
> > > interpretation of Wikipedia indicates that it should be possible to
> > > create a MBR disk with 4K sector size.  Maybe I'm doing something
> > > wrong, or parted just doesn't support this case.
> >
> > I seem to remember that 4k native disks require GPT, but if you say you
> > read otherwise, I'm not 100% sure about this any more.
> >
> > > So it appears that we should just have one blocksize control (maybe
> > > called "sectorsize"?) which sets both physical_block_size and
> > > logical_block_size to the same value.  It may also be worth enforcing
> > > that blocksize/sectorsize must be set to 512 or 4096 (which we can
> > > relax later if necessary).
> >
> > A single option (to control logical_block_size) makes sense for
> > libguestfs. physical_block_size is only relevant for the appliance and
> > not for the resulting image, so it can be treated as an implementation
> > detail.
> >
> > Kevin
> >
> 
> So, can we summarize?
> 
> - in libguestfs we will expose the only parameter called 'blocksize'
> - 512 and 4096 are the only allowed values for 'blocksize' for now

... and unset, which means 512.

> - we will reject libvirt XML with values for physical_* and
>   logical_block_size other then 512 or 4096
> - importing disks configuration from libvirt XML we will use logical_block_size
> 
> Richard, are we fine with that?

Yup, looks good to me, thanks for investigating this issue.

Rich.

-- 
Richard Jones, Virtualization Group, Red Hat http://people.redhat.com/~rjones
Read my programming and virtualization blog: http://rwmj.wordpress.com
virt-df lists disk usage of guests without needing to install any
software inside the virtual machine.  Supports Linux and Windows.
http://people.redhat.com/~rjones/virt-df/

Re: [Libguestfs] [RFC] lib: allow to specify physical/logical block size for disks

[Paolo Bonzini]

On 10/02/20 12:43, Richard W.M. Jones wrote:
> So it appears that we should just have one blocksize control (maybe
> called "sectorsize"?) which sets both physical_block_size and
> logical_block_size to the same value.  It may also be worth enforcing
> that blocksize/sectorsize must be set to 512 or 4096 (which we can
> relax later if necessary).

Yes, physical_sector_size is basically a hint to the OS and is of rather
limited usefulness.

Paolo