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E/S atómica y transaccional con libjio

Alberto Bertogli
(albertogli@telpin.com.ar)

17/Julio/2004

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Escrituras

	Asincrónicas
		Page Cache / Writeback
		HD Write Caching
	Sincrónicas
		Velocidad - Tiempo de búsqueda
		Garantías a nivel filesystem
	Concurrencia
		Escrituras solapadas
		File locks
	Fallas inesperadas
		Interrupciones abruptas
		Corrupción de archivos
		Tiempos de recuperación

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Atomicidad

	Operaciones indivisibles
	Necesaria para sistemas confiables
	MTAs, DBs, embebidos, etc.
	Garantías existentes en los filesystems POSIX
		No hay operaciones atómicas
		No hay estandarización
		No hay garantías de ordenamiento
		Si hay fsync()

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Transacciones

	Agrupamiento ordenado de operaciones
	Commit
	Rollback
	Garantías transaccionales (ACID)
		Atomicidad
		Consistencia
		Aislamiento
		Durabilidad

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libjio

	En userspace
	Transacciones atómicas de E/S
	Permite trabajo concurrente
	Locks de escritura y lectura diferenciados
	Bajos tiempos de recuperación
	Portable e independiente del filesystem
	Embebible (aprox. 9k)
	APIs similares a UNIX y a ANSI I/O

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Formato en el disco

	Archivo original intacto
	Directorio de journal
	Un archivo por cada transacción en curso
		Header principal
		Operaciones (header de op. + datos)
		Checksum

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Commit

	Bloquear las porciones del archivo donde se va a trabajar
	Crear el archivo de transacción
	Leer los datos que están actualmente en el archivo
	Escribir los datos en el archivo de transacción
	Escribir los datos al archivo
	Marcar la transacción como aplicada
	Borrar el archivo de transacción
	Desbloquear las porciones bloqueadas al principio

jtrans_init(file, trans);
jtrans_add(trans, buf, len, offset);
...
jtrans_commit(trans);

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Rollback

	Crear una nueva transacción igual a la que queremos deshacer
	Intercambiar los datos previos con los nuevos
	Hacer un commit de la nueva transacción

jtrans_rollback(trans);

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Recuperación ante fallas

	"Camina" el directorio de journal
	Verifica la integridad de las transacciones
	Reaplica las transacciones completas
	Limpia el journal

jfsck(filename, result);
jfsck_cleanup(filename);

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Transacciones "vagas"

	Utilizan el cache del OS
	Realizan solo una escritura sincrónica
	Las escrituras fisicas al archivo real son diferidas
	Mejor performance
	Mayor tiempo de recuperación
	Mayor espacio en disco

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API

jopen(); jclose(); jsync();

jtrans_init(); jtrans_free();
jtrans_add(); jtrans_commit(); jtrans_rollback();

jread(); jwrite();
jreadv(); jwritev();
jpread(); jpwrite();
jtruncate();

jfsck(); jfsck_cleanup();