[PATCH] mkfs: Emit a clearer message if too-small log is due to AG geometry

[PATCH] mkfs: Emit a clearer message if too-small log is due to AG geometry

[Eric Sandeen]

There is a constraint in mkfs.xfs that logs must not be smaller than
64MiB. We can arrive at a too-small log without specifying its size,
though, if we specify so many AGs (or such small AGs) that a single
AG cannot hold the minimum-sized log. This leads to confusing output:

$ truncate --size=500m testfile
$ mkfs.xfs -dagcount=10 testfile
Log size must be at least 64MiB.
Usage: mkfs.xfs
...

because the user did not specify the log size at all! If this error
occurs due to the AG geometry specification, make that clear with a
better error message.

Signed-off-by: Eric Sandeen <sandeen@redhat.com>
---
 mkfs/xfs_mkfs.c | 19 ++++++++++++++++++-
 1 file changed, 18 insertions(+), 1 deletion(-)

diff --git a/mkfs/xfs_mkfs.c b/mkfs/xfs_mkfs.c
index a4864c37..0c72422c 100644
--- a/mkfs/xfs_mkfs.c
+++ b/mkfs/xfs_mkfs.c
@@ -3456,8 +3456,25 @@ validate_supported(
 	 */
 	if (mp->m_sb.sb_logblocks <
 			XFS_MIN_REALISTIC_LOG_BLOCKS(mp->m_sb.sb_blocklog)) {
-		fprintf(stderr,
+		/*
+		 * An internal log must fit within a single allocation group.
+		 * If the user constrained the AG geometry but didn't ask for a
+		 * specific log size, the undersized log is a consequence of
+		 * allocation groups that are too small, not a log size the user
+		 * chose.  Point them at the real cause.
+		 */
+		if (!cli_opt_set(&lopts, L_SIZE) &&
+		    (cli_opt_set(&dopts, D_AGCOUNT) ||
+		     cli_opt_set(&dopts, D_AGSIZE))) {
+			fprintf(stderr,
+ _("Allocation group size (%lld MiB) is too small to hold the minimum 64MiB log.\n"
+   "Specify fewer or larger allocation groups, or use a larger data device.\n"),
+				((long long)mp->m_sb.sb_agblocks <<
+					mp->m_sb.sb_blocklog) >> 20);
+		} else {
+			fprintf(stderr,
  _("Log size must be at least 64MiB.\n"));
+		}
 		usage();
 	}
 
-- 
2.55.0

[PATCH V2] mkfs: Emit a clearer message if too-small log is due to AG geometry

[Eric Sandeen]

There is a constraint in mkfs.xfs that logs must not be smaller than
64MiB. We can arrive at a too-small log without specifying its size,
though, if we specify so many AGs (or such small AGs) that a single
AG cannot hold the minimum-sized log. This leads to confusing output:

$ truncate --size=500m testfile
$ mkfs.xfs -dagcount=10 testfile
Log size must be at least 64MiB.
Usage: mkfs.xfs
...

because the user did not specify the log size at all! If this error
occurs due to the AG geometry specification, make that clear with a
better error message.

Signed-off-by: Eric Sandeen <sandeen@redhat.com>
---
 mkfs/xfs_mkfs.c | 19 ++++++++++++++++++-
 1 file changed, 18 insertions(+), 1 deletion(-)

V2: Ugh as soon as I sent this I realized I forgot to restrict this to internal
logs. An external log may be too small all on its own, unrelated to AG size.

diff --git a/mkfs/xfs_mkfs.c b/mkfs/xfs_mkfs.c
index a4864c37..4f11c9de 100644
--- a/mkfs/xfs_mkfs.c
+++ b/mkfs/xfs_mkfs.c
@@ -3456,8 +3456,25 @@ validate_supported(
 	 */
 	if (mp->m_sb.sb_logblocks <
 			XFS_MIN_REALISTIC_LOG_BLOCKS(mp->m_sb.sb_blocklog)) {
-		fprintf(stderr,
+		/*
+		 * An internal log must fit within a single allocation group.
+		 * If the user specified the AG geometry but didn't ask for a
+		 * specific log size, the undersized log is due to allocation
+		 * groups that are too small, not a log size the user chose.
+		 */
+		if (cli->loginternal &&
+		    !cli_opt_set(&lopts, L_SIZE) &&
+		    (cli_opt_set(&dopts, D_AGCOUNT) ||
+		     cli_opt_set(&dopts, D_AGSIZE))) {
+			fprintf(stderr,
+ _("Allocation group size (%lld MiB) is too small to hold the minimum 64MiB log.\n"
+   "Specify fewer or larger allocation groups, or use a larger data device.\n"),
+				(long long)XFS_FSB_TO_B(mp,
+					mp->m_sb.sb_agblocks) >> 20);
+		} else {
+			fprintf(stderr,
  _("Log size must be at least 64MiB.\n"));
+		}
 		usage();
 	}
 
-- 
2.55.0

Re: [PATCH V2] mkfs: Emit a clearer message if too-small log is due to AG geometry

[Darrick J. Wong]

On Wed, Aug 26, 2026 at 10:49:38PM -0500, Eric Sandeen wrote:
> There is a constraint in mkfs.xfs that logs must not be smaller than
> 64MiB. We can arrive at a too-small log without specifying its size,
> though, if we specify so many AGs (or such small AGs) that a single
> AG cannot hold the minimum-sized log. This leads to confusing output:
> 
> $ truncate --size=500m testfile
> $ mkfs.xfs -dagcount=10 testfile
> Log size must be at least 64MiB.
> Usage: mkfs.xfs
> ...
> 
> because the user did not specify the log size at all! If this error
> occurs due to the AG geometry specification, make that clear with a
> better error message.
> 
> Signed-off-by: Eric Sandeen <sandeen@redhat.com>
> ---
>  mkfs/xfs_mkfs.c | 19 ++++++++++++++++++-
>  1 file changed, 18 insertions(+), 1 deletion(-)
> 
> V2: Ugh as soon as I sent this I realized I forgot to restrict this to internal
> logs. An external log may be too small all on its own, unrelated to AG size.
> 
> diff --git a/mkfs/xfs_mkfs.c b/mkfs/xfs_mkfs.c
> index a4864c37..4f11c9de 100644
> --- a/mkfs/xfs_mkfs.c
> +++ b/mkfs/xfs_mkfs.c
> @@ -3456,8 +3456,25 @@ validate_supported(
>  	 */
>  	if (mp->m_sb.sb_logblocks <
>  			XFS_MIN_REALISTIC_LOG_BLOCKS(mp->m_sb.sb_blocklog)) {
> -		fprintf(stderr,
> +		/*
> +		 * An internal log must fit within a single allocation group.
> +		 * If the user specified the AG geometry but didn't ask for a
> +		 * specific log size, the undersized log is due to allocation
> +		 * groups that are too small, not a log size the user chose.
> +		 */
> +		if (cli->loginternal &&
> +		    !cli_opt_set(&lopts, L_SIZE) &&
> +		    (cli_opt_set(&dopts, D_AGCOUNT) ||
> +		     cli_opt_set(&dopts, D_AGSIZE))) {
> +			fprintf(stderr,
> + _("Allocation group size (%lld MiB) is too small to hold the minimum 64MiB log.\n"
> +   "Specify fewer or larger allocation groups, or use a larger data device.\n"),

I like this!  With a couple of comments:

Can the warning message be more targeted to the option?  e.g.

	if (loginternal && !set(L_SIZE)) {
		if (set(D_AGCOUNT))
			fprintf("Specify fewer AGs or a larger data device...");
		else
			fprintf("Specify larger allocation groups...");
	}

Also, what do you think about not hardcoding 64MiB?

printf("Log size must be at least %uMiB\n",
		XFS_MIN_REALISTIC_LOG_BLOCKS(20));

(separate change obviously)

--D

> +				(long long)XFS_FSB_TO_B(mp,
> +					mp->m_sb.sb_agblocks) >> 20);
> +		} else {
> +			fprintf(stderr,
>   _("Log size must be at least 64MiB.\n"));
> +		}
>  		usage();
>  	}
>  
> -- 
> 2.55.0
> 
> 

Re: [PATCH V2] mkfs: Emit a clearer message if too-small log is due to AG geometry

[Eric Sandeen]

On 8/27/26 12:01 AM, Darrick J. Wong wrote:
> On Wed, Aug 26, 2026 at 10:49:38PM -0500, Eric Sandeen wrote:
>> There is a constraint in mkfs.xfs that logs must not be smaller than
>> 64MiB. We can arrive at a too-small log without specifying its size,
>> though, if we specify so many AGs (or such small AGs) that a single
>> AG cannot hold the minimum-sized log. This leads to confusing output:
>>
>> $ truncate --size=500m testfile
>> $ mkfs.xfs -dagcount=10 testfile
>> Log size must be at least 64MiB.
>> Usage: mkfs.xfs
>> ...
>>
>> because the user did not specify the log size at all! If this error
>> occurs due to the AG geometry specification, make that clear with a
>> better error message.
>>
>> Signed-off-by: Eric Sandeen <sandeen@redhat.com>
>> ---
>>  mkfs/xfs_mkfs.c | 19 ++++++++++++++++++-
>>  1 file changed, 18 insertions(+), 1 deletion(-)
>>
>> V2: Ugh as soon as I sent this I realized I forgot to restrict this to internal
>> logs. An external log may be too small all on its own, unrelated to AG size.
>>
>> diff --git a/mkfs/xfs_mkfs.c b/mkfs/xfs_mkfs.c
>> index a4864c37..4f11c9de 100644
>> --- a/mkfs/xfs_mkfs.c
>> +++ b/mkfs/xfs_mkfs.c
>> @@ -3456,8 +3456,25 @@ validate_supported(
>>  	 */
>>  	if (mp->m_sb.sb_logblocks <
>>  			XFS_MIN_REALISTIC_LOG_BLOCKS(mp->m_sb.sb_blocklog)) {
>> -		fprintf(stderr,
>> +		/*
>> +		 * An internal log must fit within a single allocation group.
>> +		 * If the user specified the AG geometry but didn't ask for a
>> +		 * specific log size, the undersized log is due to allocation
>> +		 * groups that are too small, not a log size the user chose.
>> +		 */
>> +		if (cli->loginternal &&
>> +		    !cli_opt_set(&lopts, L_SIZE) &&
>> +		    (cli_opt_set(&dopts, D_AGCOUNT) ||
>> +		     cli_opt_set(&dopts, D_AGSIZE))) {
>> +			fprintf(stderr,
>> + _("Allocation group size (%lld MiB) is too small to hold the minimum 64MiB log.\n"
>> +   "Specify fewer or larger allocation groups, or use a larger data device.\n"),
> 
> I like this!  With a couple of comments:
> 
> Can the warning message be more targeted to the option?  e.g.
> 
> 	if (loginternal && !set(L_SIZE)) {
> 		if (set(D_AGCOUNT))
> 			fprintf("Specify fewer AGs or a larger data device...");
> 		else
> 			fprintf("Specify larger allocation groups...");
> 	}

And if the user specified both size and count? I'm not really convinced
that we need to go that far, I guess. All the logic in that block is
kind of eye-bleeding already IMHO.

> Also, what do you think about not hardcoding 64MiB?
> 
> printf("Log size must be at least %uMiB\n",
> 		XFS_MIN_REALISTIC_LOG_BLOCKS(20));

That give blocks, not MiB I think, right? 
> (separate change obviously)

Yeah I noticed that but was just following djwong's lead from the existing
printf. ;) 

#define XFS_MIN_REALISTIC_LOG_BLOCKS(blog)      (MEGABLOCKS(64, (blog)))

Maybe we'd need:

#define XFS_MIN_REALISTIC_LOG_MB	64

and then

#define XFS_MIN_REALISTIC_LOG_BLOCKS(blog)      (MEGABLOCKS(XFS_MIN_REALISTIC_LOG_MB, (blog)))

and then

printf("Log size must be at least %uMiB\n", XFS_MIN_REALISTIC_LOG_MB);

or something?

-Eric

> --D
> 
>> +				(long long)XFS_FSB_TO_B(mp,
>> +					mp->m_sb.sb_agblocks) >> 20);
>> +		} else {
>> +			fprintf(stderr,
>>   _("Log size must be at least 64MiB.\n"));
>> +		}
>>  		usage();
>>  	}
>>  
>> -- 
>> 2.55.0
>>
>>
> 

Re: [PATCH V2] mkfs: Emit a clearer message if too-small log is due to AG geometry

[Darrick J. Wong]

On Thu, Aug 27, 2026 at 08:19:52AM -0500, Eric Sandeen wrote:
> On 8/27/26 12:01 AM, Darrick J. Wong wrote:
> > On Wed, Aug 26, 2026 at 10:49:38PM -0500, Eric Sandeen wrote:
> >> There is a constraint in mkfs.xfs that logs must not be smaller than
> >> 64MiB. We can arrive at a too-small log without specifying its size,
> >> though, if we specify so many AGs (or such small AGs) that a single
> >> AG cannot hold the minimum-sized log. This leads to confusing output:
> >>
> >> $ truncate --size=500m testfile
> >> $ mkfs.xfs -dagcount=10 testfile
> >> Log size must be at least 64MiB.
> >> Usage: mkfs.xfs
> >> ...
> >>
> >> because the user did not specify the log size at all! If this error
> >> occurs due to the AG geometry specification, make that clear with a
> >> better error message.
> >>
> >> Signed-off-by: Eric Sandeen <sandeen@redhat.com>
> >> ---
> >>  mkfs/xfs_mkfs.c | 19 ++++++++++++++++++-
> >>  1 file changed, 18 insertions(+), 1 deletion(-)
> >>
> >> V2: Ugh as soon as I sent this I realized I forgot to restrict this to internal
> >> logs. An external log may be too small all on its own, unrelated to AG size.
> >>
> >> diff --git a/mkfs/xfs_mkfs.c b/mkfs/xfs_mkfs.c
> >> index a4864c37..4f11c9de 100644
> >> --- a/mkfs/xfs_mkfs.c
> >> +++ b/mkfs/xfs_mkfs.c
> >> @@ -3456,8 +3456,25 @@ validate_supported(
> >>  	 */
> >>  	if (mp->m_sb.sb_logblocks <
> >>  			XFS_MIN_REALISTIC_LOG_BLOCKS(mp->m_sb.sb_blocklog)) {
> >> -		fprintf(stderr,
> >> +		/*
> >> +		 * An internal log must fit within a single allocation group.
> >> +		 * If the user specified the AG geometry but didn't ask for a
> >> +		 * specific log size, the undersized log is due to allocation
> >> +		 * groups that are too small, not a log size the user chose.
> >> +		 */
> >> +		if (cli->loginternal &&
> >> +		    !cli_opt_set(&lopts, L_SIZE) &&
> >> +		    (cli_opt_set(&dopts, D_AGCOUNT) ||
> >> +		     cli_opt_set(&dopts, D_AGSIZE))) {
> >> +			fprintf(stderr,
> >> + _("Allocation group size (%lld MiB) is too small to hold the minimum 64MiB log.\n"
> >> +   "Specify fewer or larger allocation groups, or use a larger data device.\n"),
> > 
> > I like this!  With a couple of comments:
> > 
> > Can the warning message be more targeted to the option?  e.g.
> > 
> > 	if (loginternal && !set(L_SIZE)) {
> > 		if (set(D_AGCOUNT))
> > 			fprintf("Specify fewer AGs or a larger data device...");
> > 		else
> > 			fprintf("Specify larger allocation groups...");
> > 	}
> 
> And if the user specified both size and count? I'm not really convinced
> that we need to go that far, I guess. All the logic in that block is
> kind of eye-bleeding already IMHO.

Eh, ok.  I forgot that maybe the user did both, so they get both
messages. :)

Reviewed-by: "Darrick J. Wong" <djwong@kernel.org>

--D


> > Also, what do you think about not hardcoding 64MiB?
> > 
> > printf("Log size must be at least %uMiB\n",
> > 		XFS_MIN_REALISTIC_LOG_BLOCKS(20));
> 
> That give blocks, not MiB I think, right? 
> > (separate change obviously)
> 
> Yeah I noticed that but was just following djwong's lead from the existing
> printf. ;) 

Yeah but djwong is laaaaazy.

> #define XFS_MIN_REALISTIC_LOG_BLOCKS(blog)      (MEGABLOCKS(64, (blog)))
> 
> Maybe we'd need:
> 
> #define XFS_MIN_REALISTIC_LOG_MB	64
> 
> and then
> 
> #define XFS_MIN_REALISTIC_LOG_BLOCKS(blog)      (MEGABLOCKS(XFS_MIN_REALISTIC_LOG_MB, (blog)))
> 
> and then
> 
> printf("Log size must be at least %uMiB\n", XFS_MIN_REALISTIC_LOG_MB);
> 
> or something?

MEGABLOCKS(xx, 20) gets you megabytes:

#define MEGABLOCKS(count, blog)      ((uint64_t)(count) << (20 - (blog)))

so XFS_MIN_REALISTIC_LOG_BLOCKS(20) gets you the min realistic log size
in megabytes.

#define XFS_MIN_REALISTIC_LOG_MB \
	(XFS_MIN_REALISTIC_LOG_BLOCKS(20))

Your way also works. :)

--D

> -Eric
> 
> > --D
> > 
> >> +				(long long)XFS_FSB_TO_B(mp,
> >> +					mp->m_sb.sb_agblocks) >> 20);
> >> +		} else {
> >> +			fprintf(stderr,
> >>   _("Log size must be at least 64MiB.\n"));
> >> +		}
> >>  		usage();
> >>  	}
> >>  
> >> -- 
> >> 2.55.0
> >>
> >>
> > 
>