MTIO(4)

💅 MTIO(4) — the FreeBSD magtape interface, explained by your unix bestie 💅

okay so this is mtio(4) from the Kernel Interfaces Manual, and it's literally the whole vibe of how FreeBSD talks to magnetic tape drives. it's giving "niche but iconic." strap in 🎀

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✨ NAME ✨

mtio — the FreeBSD magtape interface 🦋

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🌸 DESCRIPTION 🌸

so the special files named /dev/[en]sa* refer to SCSI tape drives, which may be attached to the system. no cap. and /dev/sa*.ctl are control devices — you use these to issue ioctls to the SCSI tape driver to set parameters that are required to last beyond the unmounting of a tape. like, they outlive the unmount, main character energy 👑

the rewind devices automatically rewind when the last requested read, write or seek has finished, or when the end of the tape has been reached. the letter ‘n’ is prepended to the name of the no-rewind devices. the letter ‘e’ is prepended to the name of the eject devices. that's the whole naming lore, iykyk 💅

tapes can be written with either fixed length records or variable length records. see sa(4) for more info, bestie. two filemarks mark the end of a tape, and one filemark marks the end of a tape file. and if the tape is not to be rewound, it is positioned with the head in between the two tape marks, where the next write will over write the second end-of-file marker. lowkey unhinged but that's how it works 🌀

all of the magtape devices may be manipulated with the mt(1) command. use the tool, love the tool 🫶

a number of ioctl(2) operations are available on raw magnetic tape. the following definitions are from <sys/mtio.h>. literally here they are, verbatim, don't touch ✨

#ifndef _SYS_MTIO_H_
#define _SYS_MTIO_H_

#ifndef _KERNEL
#include <sys/types.h>
#endif
#include <sys/ioccom.h>

/*
 * Structures and definitions for mag tape io control commands
 */

/* structure for MTIOCTOP - mag tape op command */
struct mtop {
        short   mt_op;          /* operations defined below */
        int32_t mt_count;       /* how many of them */
};

/* operations */
#define MTWEOF          0       /* write an end-of-file record */
#define MTFSF           1       /* forward space file */
#define MTBSF           2       /* backward space file */
#define MTFSR           3       /* forward space record */
#define MTBSR           4       /* backward space record */
#define MTREW           5       /* rewind */
#define MTOFFL          6       /* rewind and put the drive offline */
#define MTNOP           7       /* no operation, sets status only */
#define MTCACHE         8       /* enable controller cache */
#define MTNOCACHE       9       /* disable controller cache */

that struct mtop is the little duo that runs the whole op game — mt_op says what, mt_count says how many of them. iconic pairing honestly 💖

and then the FreeBSD-only bit, which is where it gets spicy 🔥

#if defined(__FreeBSD__)
/* Set block size for device. If device is a variable size dev          */
/* a non zero parameter will change the device to a fixed block size    */
/* device with block size set to that of the parameter passed in.       */
/* Resetting the block size to 0 will restore the device to a variable  */
/* block size device. */

#define MTSETBSIZ       10

/* Set density values for device. Sets the value for the opened mode only. */

#define MTSETDNSTY      11

#define MTERASE         12      /* erase to EOM */
#define MTEOD           13      /* Space to EOM */
#define MTCOMP          14      /* select compression mode 0=off, 1=def */
#define MTRETENS        15      /* re-tension tape */
#define MTWSS           16      /* write setmark(s) */
#define MTFSS           17      /* forward space setmark */
#define MTBSS           18      /* backward space setmark */
#define MTLOAD          19      /* load tape in drive */
#define MTWEOFI         20      /* write an end-of-file record without waiting*/

#define MT_COMP_ENABLE          0xffffffff
#define MT_COMP_DISABLED        0xfffffffe
#define MT_COMP_UNSUPP          0xfffffffd

note that MTSETDNSTY sets the value for the opened mode only — don't forget that part, it's the fine print bestie 🎀

next up, the values in mt_dsreg that say what the device is doing. it's like a mood ring but for your tape drive 🦋

/*
 * Values in mt_dsreg that say what the device is doing
 */
#define MTIO_DSREG_NIL  0       /* Unknown */
#define MTIO_DSREG_REST 1       /* Doing Nothing */
#define MTIO_DSREG_RBSY 2       /* Communicating with tape (but no motion) */
#define MTIO_DSREG_WR   20      /* Writing */
#define MTIO_DSREG_FMK  21      /* Writing Filemarks */
#define MTIO_DSREG_ZER  22      /* Erasing */
#define MTIO_DSREG_RD   30      /* Reading */
#define MTIO_DSREG_FWD  40      /* Spacing Forward */
#define MTIO_DSREG_REV  41      /* Spacing Reverse */
#define MTIO_DSREG_POS  42      /* Hardware Positioning (direction unknown) */
#define MTIO_DSREG_REW  43      /* Rewinding */
#define MTIO_DSREG_TEN  44      /* Retensioning */
#define MTIO_DSREG_UNL  45      /* Unloading */
#define MTIO_DSREG_LD   46      /* Loading */

#endif  /* __FreeBSD__ */

MTIO_DSREG_RBSY is "communicating with tape (but no motion)" which is SO real, that's me texting back and saying nothing 😭

now the good stuff — the get-status struct 🌟

/* structure for MTIOCGET - mag tape get status command */

struct mtget {
        short   mt_type;        /* type of magtape device */
/* the following two registers are grossly device dependent */
        short   mt_dsreg;       /* ``drive status'' register */
        short   mt_erreg;       /* ``error'' register */
/* end device-dependent registers */
        /*
         * Note that the residual count, while maintained, may be
         * be nonsense because the size of the residual may (greatly)
         * exceed 32 K-bytes. Use the MTIOCERRSTAT ioctl to get a
         * more accurate count.
         */
        short   mt_resid;       /* residual count */
#if defined (__FreeBSD__)
        int32_t mt_blksiz;      /* presently operating blocksize */
        int32_t mt_density;     /* presently operating density */
        uint32_t mt_comp;       /* presently operating compression */
        int32_t mt_blksiz0;     /* blocksize for mode 0 */
        int32_t mt_blksiz1;     /* blocksize for mode 1 */
        int32_t mt_blksiz2;     /* blocksize for mode 2 */
        int32_t mt_blksiz3;     /* blocksize for mode 3 */
        int32_t mt_density0;    /* density for mode 0 */
        int32_t mt_density1;    /* density for mode 1 */
        int32_t mt_density2;    /* density for mode 2 */
        int32_t mt_density3;    /* density for mode 3 */
/* the following are not yet implemented */
        uint32_t mt_comp0;      /* compression type for mode 0 */
        uint32_t mt_comp1;      /* compression type for mode 1 */
        uint32_t mt_comp2;      /* compression type for mode 2 */
        uint32_t mt_comp3;      /* compression type for mode 3 */
/* end not yet implemented */
#endif
        int32_t mt_fileno;      /* relative file number of current position */
        int32_t mt_blkno;       /* relative block number of current position */
};

🚨 real talk: the residual count, while maintained, may be nonsense because the size of the residual may (greatly) exceed 32 K-bytes. use the MTIOCERRSTAT ioctl to get a more accurate count. this is a genuine caveat, not me being dramatic. well, it's both 💀

also peep that mt_comp0 through mt_comp3 are marked not yet implemented — the space is reserved, the dreams are big 🧚

next, the error-status struct. this one is really only supported for SCSI tapes right now, so if you're on something else, respectfully, this isn't for you 🙈

/* structure for MTIOCERRSTAT - tape get error status command */
/* really only supported for SCSI tapes right now */
struct scsi_tape_errors {
        /*
         * These are latched from the last command that had a SCSI
         * Check Condition noted for these operations. The act
         * of issuing an MTIOCERRSTAT unlatches and clears them.
         */
        uint8_t io_sense[32];   /* Last Sense Data For Data I/O */
        int32_t io_resid;       /* residual count from last Data I/O */
        uint8_t io_cdb[16];     /* Command that Caused the Last Data Sense */
        uint8_t ctl_sense[32];  /* Last Sense Data For Control I/O */
        int32_t ctl_resid;      /* residual count from last Control I/O */
        uint8_t ctl_cdb[16];    /* Command that Caused the Last Control Sense */
        /*
         * These are the read and write cumulative error counters.
         * (how to reset cumulative error counters is not yet defined).
         * (not implemented as yet but space is being reserved for them)
         */
        struct {
                uint32_t retries;       /* total # retries performed */
                uint32_t corrected;     /* total # corrections performed */
                uint32_t processed;     /* total # corrections successful */
                uint32_t failures;      /* total # corrections/retries failed */
                uint64_t nbytes;        /* total # bytes processed */
        } wterr, rderr;
};

union mterrstat {
        struct scsi_tape_errors scsi_errstat;
        char _reserved_padding[256];
};

important detail bestie: those first fields are latched from the last command that had a SCSI Check Condition noted for these operations — and the act of issuing an MTIOCERRSTAT unlatches and clears them. reading clears. one look and it's gone 😭

also: how to reset the cumulative error counters is not yet defined, and they're not implemented as yet but space is being reserved for them. we love a reserved seat 🎀

and then the block-limit struct, no commentary needed, it's just vibes 🌈

struct mtrblim {
        uint32_t granularity;
        uint32_t min_block_length;
        uint32_t max_block_length;
};

now the locate situation — enums and a struct, all together now ⭐

typedef enum {
        MT_LOCATE_DEST_OBJECT   = 0x00,
        MT_LOCATE_DEST_FILE     = 0x01,
        MT_LOCATE_DEST_SET      = 0x02,
        MT_LOCATE_DEST_EOD      = 0x03
} mt_locate_dest_type;

typedef enum {
        MT_LOCATE_BAM_IMPLICIT  = 0x00,
        MT_LOCATE_BAM_EXPLICIT  = 0x01
} mt_locate_bam;

typedef enum {
        MT_LOCATE_FLAG_IMMED            = 0x01,
        MT_LOCATE_FLAG_CHANGE_PART      = 0x02
} mt_locate_flags;

struct mtlocate {
        mt_locate_flags         flags;
        mt_locate_dest_type     dest_type;
        mt_locate_bam           block_address_mode;
        int64_t                 partition;
        uint64_t                logical_id;
        uint8_t                 reserved[64];
};

then the extended-get stuff, which is giving "there's more but you gotta ask nicely" 💅

typedef enum {
        MT_EXT_GET_NONE,
        MT_EXT_GET_OK,
        MT_EXT_GET_NEED_MORE_SPACE,
        MT_EXT_GET_ERROR
} mt_ext_get_status;

struct mtextget {
        uint32_t                alloc_len;
        char                    *status_xml;
        uint32_t                fill_len;
        mt_ext_get_status       status;
        char                    error_str[128];
        uint8_t                 reserved[64];
};

#define MT_EXT_GET_ROOT_NAME            "mtextget"
#define MT_DENSITY_ROOT_NAME            "mtdensity"
#define MT_MEDIA_DENSITY_NAME           "media_density"
#define MT_DENSITY_REPORT_NAME          "density_report"
#define MT_MEDIUM_TYPE_REPORT_NAME      "medium_type_report"
#define MT_MEDIA_REPORT_NAME            "media_report"
#define MT_DENSITY_ENTRY_NAME           "density_entry"

#define MT_DENS_WRITE_OK                0x80
#define MT_DENS_DUP                     0x40
#define MT_DENS_DEFLT                   0x20

#define MT_PARAM_FIXED_STR_LEN  32
union mt_param_value {
        int64_t         value_signed;
        uint64_t        value_unsigned;
        char            *value_var_str;
        char            value_fixed_str[MT_PARAM_FIXED_STR_LEN];
        uint8_t         reserved[64];
};

typedef enum {
        MT_PARAM_SET_NONE,
        MT_PARAM_SET_SIGNED,
        MT_PARAM_SET_UNSIGNED,
        MT_PARAM_SET_VAR_STR,
        MT_PARAM_SET_FIXED_STR
} mt_param_set_type;

typedef enum {
        MT_PARAM_STATUS_NONE,
        MT_PARAM_STATUS_OK,
        MT_PARAM_STATUS_ERROR
} mt_param_set_status;

and that's where this part lands — value_fixed_str is MT_PARAM_FIXED_STR_LEN (that's 32, keep up bestie) and there's a whole reserved[64] of future potential sitting there like a group chat with no messages yet 🦋

So we're still deep in the header file, bestie — here's the rest of it, and honestly it's giving ancient scroll of tape wisdom 📜✨. Every line copied exactly, because we do not mess with #defines:

#define MT_PARAM_VALUE_NAME_LEN 64
struct mtparamset {
        char                    value_name[MT_PARAM_VALUE_NAME_LEN];
        mt_param_set_type       value_type;
        int                     value_len;
        union mt_param_value    value;
        mt_param_set_status     status;
        char                    error_str[128];
};

#define MT_PARAM_ROOT_NAME      "mtparamget"
#define MT_PROTECTION_NAME      "protection"

/*
 * Set a list of parameters.
 */
struct mtsetlist {
        int num_params;
        int param_len;
        struct mtparamset *params;
};

Like, error_str[128]? She's ready to tell you exactly what went wrong and she has space to do it 💅. And mtsetlist is just "a bunch of params, please, all at once" — a girl who batch-processes 🎀.

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Now the part everyone screenshots: the mt_type constants 🔥. These are the same for controllers that are compatible with the types listed — keep that in mind, iykyk:

/*
 * Constants for mt_type byte.  These are the same
 * for controllers compatible with the types listed.
 */
#define MT_ISTS         0x01            /* TS-11 */
#define MT_ISHT         0x02            /* TM03 Massbus: TE16, TU45, TU77 */
#define MT_ISTM         0x03            /* TM11/TE10 Unibus */
#define MT_ISMT         0x04            /* TM78/TU78 Massbus */
#define MT_ISUT         0x05            /* SI TU-45 emulation on Unibus */
#define MT_ISCPC        0x06            /* SUN */
#define MT_ISAR         0x07            /* SUN */
#define MT_ISTMSCP      0x08            /* DEC TMSCP protocol (TU81, TK50) */
#define MT_ISCY         0x09            /* CCI Cipher */
#define MT_ISCT         0x0a            /* HP 1/4 tape */
#define MT_ISFHP        0x0b            /* HP 7980 1/2 tape */
#define MT_ISEXABYTE    0x0c            /* Exabyte */
#define MT_ISEXA8200    0x0c            /* Exabyte EXB-8200 */
#define MT_ISEXA8500    0x0d            /* Exabyte EXB-8500 */
#define MT_ISVIPER1     0x0e            /* Archive Viper-150 */
#define MT_ISPYTHON     0x0f            /* Archive Python (DAT) */
#define MT_ISHPDAT      0x10            /* HP 35450A DAT drive */
#define MT_ISMFOUR      0x11            /* M4 Data 1/2 9track drive */
#define MT_ISTK50       0x12            /* DEC SCSI TK50 */
#define MT_ISMT02       0x13            /* Emulex MT02 SCSI tape controller */

Not me spotting that MT_ISEXABYTE and MT_ISEXA8200 are both 0x0c 👀 — that's canon, that's in the source, don't come for me, I'm just the messenger 🫶.

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And here come the actual mag tape io control commands — the main characters of the whole page 👑:

/* mag tape io control commands */
#define MTIOCTOP        _IOW('m', 1, struct mtop)       /* do a mag tape op */
#define MTIOCGET        _IOR('m', 2, struct mtget)      /* get tape status */
/* these two do not appear to be used anywhere */
#define MTIOCIEOT       _IO('m', 3)                     /* ignore EOT error */
#define MTIOCEEOT       _IO('m', 4)                     /* enable EOT error */
/*
 * When more SCSI-3 SSC (streaming device) devices are out there
 * that support the full 32 byte type 2 structure, we'll have to
 * rethink these ioctls to support all the entities they haul into
 * the picture (64 bit blocks, logical file record numbers, etc..).
 */
#define MTIOCRDSPOS     _IOR('m', 5, uint32_t)  /* get logical blk addr */
#define MTIOCRDHPOS     _IOR('m', 6, uint32_t)  /* get hardware blk addr */
#define MTIOCSLOCATE    _IOW('m', 5, uint32_t)  /* seek to logical blk addr */
#define MTIOCHLOCATE    _IOW('m', 6, uint32_t)  /* seek to hardware blk addr */
#define MTIOCERRSTAT    _IOR('m', 7, union mterrstat)   /* get tape errors */
/*
 * Set EOT model- argument is number of filemarks to end a tape with.
 * Note that not all possible values will be accepted.
 */
#define MTIOCSETEOTMODEL        _IOW('m', 8, uint32_t)
/* Get current EOT model */
#define MTIOCGETEOTMODEL        _IOR('m', 8, uint32_t)
#define MTIOCRBLIM      _IOR('m', 9, struct mtrblim)    /* get block limits */
#define MTIOCEXTLOCATE  _IOW('m', 10, struct mtlocate)  /* seek to position */
#define MTIOCEXTGET     _IOWR('m', 11, struct mtextget) /* get tape status */
#define MTIOCPARAMGET   _IOWR('m', 12, struct mtextget) /* get tape params */
#define MTIOCPARAMSET   _IOWR('m', 13, struct mtparamset) /* set tape params */
#define MTIOCSETLIST    _IOWR('m', 14, struct mtsetlist) /* set N params */

Lowkey important, bestie: MTIOCIEOT and MTIOCEEOT — the source literally says "these two do not appear to be used anywhere" 😭 so like… they're invited to the party but nobody's talked to them in decades. And for MTIOCSETEOTMODEL, the comment is very clear that not all possible values will be accepted — so don't just throw numbers at it expecting a yes 💀.

And finally, the closer of the header + the default tape:

#ifndef _KERNEL
#define DEFTAPE "/dev/nsa0"
#endif

#endif /* !_SYS_MTIO_H_ */

That #endif /* !_SYS_MTIO_H_ */ is the header guard saying goodbye — iconic exit, no notes 🌈.

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🌸 FILES 🌸

• /dev/[en]sa* — where the tape devices live, slay 💖

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🦋 SEE ALSO 🦋

Go read these too, bestie, they're the friend group:

• mt(1)
• tar(1)
• sa(4)

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🎀 HISTORY 🎀

The mtio manual appeared in 4.2BSD. An i386 version first appeared in FreeBSD 2.2.

Like… she's been on the scene since 4.2BSD and she's still here, still relevant, still doing tape things. Main character energy, no cap 👑✨

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okay that's the whole page, thank you for coming to my TED talk 💅 This baby is the Debian edition, dated February 12, 2015 — big throwback vibes, and honestly the fact that a page about tape drives got updated in 2015 is so 💀/🫶 of it. Go format a tape, you've earned it, bye bestie! 🌸🦋💜